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Graduate Theses, Dissertations, and Problem Reports
2015
Design Programs for Highwall Mining Operations Design Programs for Highwall Mining Operations
Ming Fan
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Design Programs for Highwall Mining Operations
Ming Fan
Thesis submitted
to the Benjamin M.Statler College of Engineering and Mineral Resources
at West Virginia University
in partial fulfillment of the requirements for the degree of
Master of Science in
Mining Engineering
Approved by
Yi Luo, Ph.D., Chair
Brijes Mishra, Ph.D.
Felicia F. Peng, Ph.D.
Department of Mining Engineering
Morgantown, West Virginia
2015
Keywords: Highwall Mining, Safety, Health, Suitability, Highwall Failure,
Highwall Stability, Mine Roof Stability Assessment, Pillar Design Methods, Design
Optimization
Copyright 2015Ming Fan
ABSTRACT
Design Programs for Highwall Mining Operations
Ming Fan
Highwall mining is a hybrid of surface mining and underground mining methods, and is
often the only feasible method to recover the coal reserves in the central Appalachian coalfields
due to the steep terrain and the closely spaced multiple thin coal seams. Compared to the
mountain-top-removal, contour, auger, and underground mining methods, application of the
highwall mining method can reduce the environmental impacts, increase the recovery ratio of
coal reserves, and enhance mine safety as well as productivity. Therefore, it is probable that
highwall mining will be a dominant method for extracting the high-value coal resources in the
Appalachian coalfields. By far, the greatest ground control safety concerns in highwall mining
operations are rock falls from the highwall and mining equipment entrapment underground.
These hazards are most likely caused by the instability of the highwall mine system due to
insufficient mine design and difficulties encountered during mining operations.
Most of this thesis emphasizes the mine design concepts and methods to maintain the
stability of mine structures. With the purpose of evaluating the stability of the entry roof, the
beam theory is applied. After comparing the deflection, stress, and strain profiles of 0.05 ft
sandstone and mudstone roof layers, it can be concluded that the existence of relatively thin and
weak layer in the immediate roof could cause potential stability problems for the entry roof.
Therefore, for highwall mining operations to be conducted in coal seams with thinly bedded roof
strata, a correct decision to cut some of the thin weak roof rock layers with the main coal seam
can be greatly beneficial to mining operations. The pressure arch concept is applied for the
systematic design of the highwall mining operation. Within this concept, an optimization design
process is developed to improve the recovery ratio of coal resources to a reasonably high level.
For multi-seam highwall mining operations, the largest web and barrier pillar sizes should be
selected and vertically aligned into seams. Two numerical programs, Examine2D and FLAC, are
used to analyze the stability of highwall structures and to find the stable interburden thickness
where no interaction between the two coal seams is expected. Numerical results show that, under
given geology and mining conditions in the thesis, the stable interburden thickness is 20 ft and
there are no stability issues. In the end, three spreadsheet programs are developed for the
assessment of highwall mine structures, for the design of the web and barrier pillars, and for the
optimization design process, based on the proposed design concepts and methodologies.
iii
ACKNOWLEDGEMENT
Deepest appreciation is conveyed to my advisor, colleagues, friends and family for their
unrelenting support, without which this thesis could not have been turned into reality.
First of all, I would like to express my sincere gratitude and deep respect to my
supervisor Dr. Yi Luo. He is more of a mentor than a professor throughout my graduate study. I
want to thank him for his endless patience, guidance and assistance during my study at West
Virginia University.
I would also like to extend sincere thanks to Dr. Brijes Mishra for his invaluable guidance
in the construction of numerical models and constant encouragement to my research. I also
appreciate the invaluable support and suggestions from Dr. Felicia F. Peng during the research.
This work was performed under a project sponsored by the Coal and Energy Research
Bureau (CERB) of West Virginia. I gratefully acknowledge their support and sponsorship of the
research project.
I would like to give sincerest thanks and best wishes to my friends and colleagues: Chris
Vass, DanqingGao, Hua Jiang, Jian Yang, Kaifang Li, Meng Zhang, Mingming Li, Wan Wang,
Xu Tang, Xinyang Wang, YutingXue, and other graduate students in the Department of Mining
Engineering at WVU.
In the end, I want to thank all who helped me during my study in West Virginia
University. Without their support, completion of my study would have never been possible.
iv
Table of Contents
ABSTRACT ................................................................................................................................... ii
ACKNOWLEDGEMENT ........................................................................................................... iii
Table of Contents ......................................................................................................................... iiii
List of Figures ............................................................................................................................. viii
List of Tables ................................................................................................................................. xi
Chapter 1 Introduction ................................................................................................................. 1
1.1 Background ......................................................................................................................... 1
1.2 Statement of Problem ......................................................................................................... 2
1.3 Statement of Work .............................................................................................................. 2
Chapter 2 Literature Review ........................................................................................................ 4
2.1 Introduction of Highwall Mining Method ........................................................................ 4
2.1.1 Highwall Mining Method ............................................................................................ 4
2.1.2 Advantages of Highwall Mining ................................................................................. 5
2.2 Highwall Mining Process .................................................................................................... 6
2.3 Highwall Mining Systems ................................................................................................... 7
2.3.1 ADDCAR Highwall Mining System ........................................................................... 7
2.3.2 Cat HW300 Highwall Mining System ........................................................................ 9
2.4 Highwall Mining Guidance System ................................................................................. 10
2.4.1 Guidance System for ADDCAR Highwall Mining System .................................... 10
2.4.2 Guidance System for Cat HW300 Highwall Mining System ................................. 11
v
2.5 Multiple Seam Highwall Mining ...................................................................................... 12
2.6 Empirical Highwall Mining System Design .................................................................... 13
2.6.1 Coal Pillar Strength ................................................................................................... 13
2.6.2 Coal Pillar Stress ........................................................................................................ 14
2.7 Introduction of Examine2D and FLAC .......................................................................... 15
2.7.1 Methods of Numerical Modeling .............................................................................. 15
2.7.2 Introduction of Examine2D ...................................................................................... 16
2.7.3 Introduction of FLAC ............................................................................................... 17
2.7.4 Mohr-Coulomb Model ............................................................................................... 19
Chapter 3 Application of the Highwall Mining Method .......................................................... 21
3.1 Suitability of Highwall Mining in West Virginia ........................................................... 21
3.1.1 Requirements for Underground Mining .................................................................. 22
3.1.2 Limitations of Contour Mining................................................................................. 23
3.1.3 Auger Mining ............................................................................................................. 25
3.1.4 Suitability of Highwall Mining ................................................................................. 26
3.2 Main Challenges with Highwall Mining ......................................................................... 26
3.2.1 Types of Highwall ...................................................................................................... 26
3.2.2 MSHA Incident Statistics for Highwall Mining ...................................................... 27
3.2.3 Types of Highwall Failures ....................................................................................... 28
3.2.3.1 Rock Mass Failure .............................................................................................. 28
3.2.3.1.1 Planar Failures ............................................................................................ 28
vi
3.2.3.1.2 Wedge Failure ............................................................................................. 29
3.2.3.1.3 Toppling Failures ........................................................................................ 29
3.2.3.1.4 Circular Failures ......................................................................................... 31
3.2.3.2 Rock Falls ............................................................................................................ 31
3.2.3.3 Highwall Failure Examples ............................................................................... 31
3.2.4 Factors Affecting Highwall Stability ........................................................................ 34
3.2.4.1 Geologic Structure .............................................................................................. 35
3.2.4.2 Pillar Stability ..................................................................................................... 36
Chapter 4 Design Methods for Highwall Mining Operations ................................................. 39
4.1 Stability of the Highwall Top Surface ............................................................................. 39
4.2 Stability of the Highwall ................................................................................................... 39
4.3 Stability of the Entry Roof ............................................................................................... 40
4.4 Pillar Stability for Highwall Mining in Single Coal Seam ............................................. 56
4.5 Influence of Multi-Seam Mining Operations .................................................................. 58
4.6 Model Development in Examine2D ................................................................................. 59
4.7 Model Development in FLAC .......................................................................................... 63
4.7.1 Model Assumption ..................................................................................................... 63
4.7.2 Mining and Geology Conditions ............................................................................... 64
4.7.3 Boundary Conditions and Constitutive Model ........................................................ 66
4.7.4 Model Development ................................................................................................... 66
4.7.5 Calculation Analysis and Result Discussion ............................................................ 66
vii
4.8 Highwall Mining Design Optimization............................................................................ 70
4.9 Benefit of Using the Highwall Mining Design Method .................................................. 75
4.10 Highwall Mine Design Programs ................................................................................... 76
Chapter 5 Conclusions and Recommendations ........................................................................ 81
5.1 Summary ............................................................................................................................ 81
5.2 Future Research Recommendations ................................................................................ 82
Reference ...................................................................................................................................... 83
About the Author ......................................................................................................................... 87
viii
List of Figures
Figure 2 - 1 A Schematic Image of a Highwall Mining Operation (Shen, et al., 2001) ........... 5
Figure 2 - 2 ADDCAR Highwall Miner (http://www.addcarsystems.com/broad.html) .......... 8
Figure 2 - 3 Cat HW300 Miner (https://mining.cat.com/highwall-miner) ............................... 9
Figure 2 - 4 Lateral Guidance System of ADDCAR
(http://www.addcarsystems.com/techleader.html) ................................................................... 11
Figure 2 - 5 General Solution Procedure (Itasca, 2011) ........................................................... 18
Figure 2 - 6 Mohr-Coulomb Failure Criterion in FLAC (Itasca, 2011) ................................. 20
Figure 3 - 1 Geological Column at a Southern WV Mine Site (Kitts, 2009) .......................... 21
Figure 3 - 2 Typical Appalachian Ridge Top Coal Reserve (Kitts, 2009) .............................. 23
Figure 3 - 3 Contour Mine Bench Design .................................................................................. 24
Figure 3 - 4 Planar Type of Highwall Failure (MSHA, 2014) ................................................. 29
Figure 3 - 5 Wedge Type of Highwall Failure (MSHA, 2014) ................................................. 30
Figure 3 - 6 Toppling Type of Highwall Failure (MSHA, 2014) ............................................. 30
Figure 3 - 7 Circular Type of Highwall Failure (MSHA, 2014) .............................................. 31
Figure 3 - 8 Photo and Site Map of a Highwall Collapse in a Highwall Mining Operation
(MSHA, 2000) .............................................................................................................................. 32
Figure 3 - 9 Highwall Fall along Joint Interface in a Highwall Mining Operation ............... 33
Figure 3 - 10 Highwall Failure Cases in Highwall Mining Operations .................................. 34
Figure 3 - 11 Hillseams Indicated by Arrows in Contour Mine Highwall. (Note That
Weathering along Hillseam Can Extend Several Hundred Feet or More below the Surface
Zipf, 2009) .................................................................................................................................... 36
Figure 3 - 12 Highwall Collapse in Multiple Seam Mining Area (Zipf, 2009) ....................... 37
ix
Figure 4 - 1 A Beam with Fixed Ends for Assessing Mine Roof Stability in Highwall Mining
.............. …………………………………………………………………………………………..40
Figure 4 - 2 Deformation of the Soft Roof Layer above a Highwall Mine Pillar ................... 42
Figure 4 - 3 Calculation of Roof Layers above a Highwall Entry ........................................... 44
Figure 4 - 4 Input and Primary Calculation of the Roof Stability Assessment Program ..... 48
Figure 4 - 5 Roof Deflection Profiles for 0.2-ft Thick Mudstone Layer with and without
Lateral Squeezing Effect and Layers Interaction Effect.......................................................... 49
Figure 4 - 6 Roof Deflection Profiles for 0.2-ft Thick Mudstone and Sandstone Layers ...... 49
Figure 4 - 7 Roof Deflection Profiles for 0.05-ft Thick Mudstone and Sandstone Layers .... 50
Figure 4 - 8 Surface Strain Profiles of a 0.05-ft Mudstone Layer ........................................... 52
Figure 4 - 9 Surface Strain Profiles of a 0.05-ft Sandstone Layer ........................................... 52
Figure 4 - 10 Strain Profiles on a 0.2-ft Dry Mudstone Roof Layer ....................................... 53
Figure 4 - 11 Stress Profiles on the Top and Bottom Surfaces of 0.05-ft Mudstone Roof
Layer ............................................................................................................................................. 54
Figure 4 - 12 Stress Profiles on the Top and Bottom Surfaces of 0.05-ft Sandstone Roof
Layer ............................................................................................................................................. 54
Figure 4 - 13 Stress Profiles on the Top and Bottom Surfaces of 0.2-ft Mudstone Roof Layer
....................................................................................................................................................... 55
Figure 4 - 14 a Schematic for Highwall Mining Design (Luo, 2014) ....................................... 57
Figure 4 - 15 Suggested Layout for Highwall Mining in Closely Spaced Multiple Coal Seam
(Luo, 2014) .................................................................................................................................. 59
Figure 4 - 16 Failure Trajectories and Strength Factor Contours for 11.5ft Interburden
Thickness ...................................................................................................................................... 61
x
Figure 4 - 17 Failure Trajectories and Strength Factor Contours for 20ft Interburden
Thickness ...................................................................................................................................... 62
Figure 4 - 18 Failure Trajectories and Strength Factor Contours for 20ft Interburden
Thickness under Offset Upper Pillars Condition ..................................................................... 63
Figure 4 - 19 Geometry of the Model ......................................................................................... 65
Figure 4 - 20 the Geostatic State of YY-Stress Contour .......................................................... 67
Figure 4 - 21 Maximum Unbalanced Force at Initial Equilibrium ......................................... 67
Figure 4 - 22 Maximum Principal Stress around the Entries .................................................. 68
Figure 4 - 23 Minimum Principal Stress around the Entries .................................................. 69
Figure 4 - 24 Stress State around the Entries ........................................................................... 69
Figure 4 - 25 Pressure Arch in Highwall Mine Design (Luo,2014) ......................................... 72
Figure 4 - 26 Resulting Safety Factors for Web Pillars in a Production Panel ...................... 74
Figure 4 - 27 Screen Capture of the Highwall Mine Pillar Design Program ......................... 79
Figure 4 - 28 Design Optimization for Highwall Mining Panel Based on Pressure Arch
Concept ......................................................................................................................................... 80
xi
List of Tables
Table 4 - 1 Calculation Example Table ..................................................................................... 45
Table 4 - 2 Physical and Mechanical Properties of Coal Measure Rocks in Dry Condition
(Zhao) ........................................................................................................................................... 47
Table 4 - 3 Rock Mechanics Input Parameters in the Model .................................................. 65
1
Chapter 1 Introduction
1.1 Background
When the stripping ratio of surface mining operations loses profitability, the highwall
mining method can be applied to recover the coal resources beneath the final highwall (Seib,
1993; Vandergrift, et al., 2004; Schafer, 2002).Recent advances in highwall mining systems have
enhanced mining safety and increased financial gains, which makes the highwall mining an
important coal production method in the United States(Schmidt, 2015). Currently, the ADDCAR
system and the Cat HW300 are two principal highwall mining systems. These two systems are
extremely popular in the central Appalachian region (Fiscor, 2002). Due to the advanced design
and specifications of these highwall mining systems, the highwall mining method now has a
healthy share of space in the central Appalachian mining market. Compared to contour mining or
other surface mining methods, highwall mining provides many advantages, such as high
flexibility, ability to avoid geological structures, and capacity to extract still valuable resources
that are not mineable for both surface and underground mining methods. Moreover, highwall
mining operations are performed by remote control, allowing workers to work in a safe
environment free from hazards such as roof falls, gas, dust, flooding, and vehicle movements.
Therefore, highwall mining is the desired and often the only appropriate method to recover the
remaining coal resources in the Appalachian coalfields. Furthermore, due to the limitations of
other mining methods, and the tighter environmental control requirements, the highwall mining
method should be considered by more coal operators.
Highwall mining is a hybrid of surface mining and underground mining methods. With
the miner’s cutter module pushing into the coal seam through a series of push-beams, long
rectangular entries are punched into the coal seam to recover part of the remaining coal resources.
Web and barrier pillars are left to separate the entries and prevent the overburden strata from
collapsing. As the cutter head moves forward into the coal seam, fully enclosed push-beams are
inserted behind the cutter head. Then the coal is moved back to the center of the bench for
stockpiling and transport through the stacking conveyor system (Schmidt, 2015). The mining
cycle continues for 20 ft and the process is repeated until the operator reaches the desired depth.
Once the cutter head has been fully retracted, the drum is inspected and serviced. The machine is
then moved to the next entry. Furthermore, with the help of the monitoring system such as the
2
gamma detection system, predetermined amounts of coal can be cut accurately, and coal
resources can be recovered easily in soft roof or soft floor environments.
1.2 Statement of Problem
The Mine Safety and Health Administration’s (MSHA) accident and injury statistics
show that highwall mining has maintained an admirable safety record. Its fatality and injury rates
are nearly identical to the other surface mining methods, and are greatly lower than underground
mining methods. By far the greatest ground control safety concerns in highwall mining operations
are rock falls from the highwall and equipment entrapment underground. These hazards are most
likely caused by the instability of the highwall mine structure system due to insufficient mine
design or difficulties encountered during mining operations.
Generally, the factors affecting highwall stability during highwall mining operations can
be summarized into two categories, namely geological structure and highwall structure stability
(Zipf, 2005). Among the geological constraints, the hillseams are the major geological structures
that could impact or limit highwall mining. However, many precautions can be taken to minimize
the risk of failure associated with hillseams, such as skipping an entry where a hillseam enters the
highwall. Another significant concern related to the safety of highwall mining is the instability of
the highwall structure. Even though the design methods for highwall mining operations have
been proposed by National Institute of Safety and Health (NIOSH) researchers, there is still
considerable room to improve this mining method for the purpose of ensuring the stability of the
highwall structures.
It is also critical to rationally design the highwall mining system in multi-seam conditions
(Newman, 2009).In order to increase the recovery ratio of coal resources, many highwall mining
operators choose to recover multiple seams in very close proximity. However, the seams being
mined below or above a previously mined seam tend to be subjected to disturbances from the
adjacent coal seams. Therefore, multi-seam highwall mining systems should be designed with the
purpose of preventing web pillars from collapsing as well as preventing highwall failure.
1.3 Statement of Work
The objective of this research is to improve the highwall mining method to maintain the
stability of mine structures for the purposes of ensuring the safety for both personnel and mining
3
equipment. In order to systematically design a highwall mine and increase the recovery ratio of
coal reserves, three highwall mine design programs are developed.
First, this thesis studies the suitability of highwall mining in Appalachian coalfields and
main challenges with highwall mining. With regard to the particular geology and topography in
the Appalachian coalfields, some popular mining methods, namely mountain-top-removal,
contour, auger, underground, and highwall mining methods, are compared. Then, the main
challenges with highwall mining are investigated from the following aspects: types of highwall,
MSHA incident statistics for highwall mining, types of highwall failures, and the factors affecting
the highwall stability.
Second, in order to develop programs to systematically design a highwall mine, the
stability of the highwall top surface, the stability of the highwall, and the stability of the highwall
entry roof are studied. In order to assess the stability of the entry roof, the beam theory is applied.
In the meantime, the gravity effect, the squeezing effect, and the interaction effect between layers
are taken into consideration when evaluating the stability of the entry roof. Then, the pillar
stability for highwall mining in single coal seam and the influence of multi-seam mining
operations are discussed. Numerical models are constructed with Examine2D and FLAC to
illustrate examples of ways to find the minimum thickness of interburden where no interaction
between the two coal seams is expected. Additionally, the FLAC model is also performed to
check out the stability of the entry roof and pillars, and to detect other potential failure
mechanisms. Next, the pressure arch concept is applied in the panel optimization design process.
In the end, three spreadsheet programs have been developed to design highwall mine
pillars with the proposed design methodologies and concepts. The first design program can be
used to assess the roof stability if the thinly bedded weak layer exists in the immediate roof above
the coal seam. The second design program is developed to assess the pillar stability and panel
design when a single coal seam is mined based on traditional loading. In order to maximize the
recovery ratio, the third design program is developed for the panel optimization design process.
4
Chapter 2 Literature Review
2.1 Introduction of Highwall Mining Method
2.1.1 Highwall Mining Method
As mining progresses, higher stripping ratios gradually become restrictive factors for
surface mining operations. It is inevitable, at some point, for mining operations to reach an
economic threshold where the cost of overburden removal surpasses the value of the coal. When
stripping ratios lose profitability, recovery of coal resources under the highwall can be
accomplished by other means. Generally, where the ordinary open-cut mining is difficult or
impossible in a small-scale mine due to an uneconomical stripping ratio, the highwall mining
systems could be applied to extract the remaining coal reserves (Seib, 1993; Vandergrift, et al.,
2004; Schafer, 2002). Appalachian coal operators are discovering that highwall mining can be a
safe and productive method for retrieving coal from active and abandoned highwalls (Fiscor,
2002).It is gradually becoming an important mining method in the US and accounts for 4% of the
total U.S. coal production for the moment. Seventy-six highwall mining machines are currently
operating in the U.S., and there are also units cutting coal currently in Russia, Colombia, and
India. In the U.S., Appalachia is the preferred highwall home. There are approximately 62 active
highwall mining machines at work in West Virginia, Virginia, Pennsylvania, Kentucky, Ohio and
Maryland. At present, there are also operations in Alabama and Utah (Schmidt, 2015).
In the most general sense, highwall mining is the process of extracting coal reserves from
the base of an exposed highwall left by the previous surface mining. Typically, this is done by
using the remotely controlled mining methods to horizontally drive a series of nearly parallel
entries for a significant distance (Gardner, et al., 2002; Adhikary, et al., 2002). Long rectangular
entries are punched into the coal seams through the highwall operations to recover part of the
remaining coal reserves. Small pillars are left to separate the entries and to prevent the
overburden strata from collapsing. A typical modern highwall mining system is established on the
floor of an open-pit or contour bench, lying in front of the exposed coal seam at the base of the
highwall. The coal is transported from the back of the highwall miner to the surface that is
incrementally extended in length as the miner advances into the coal seam. The maximum
distance of the entry is determined by equipment design and geological conditions. Then the
equipment is pulled back from the entry, moved along the highwall, and prepared to extract
5
another entry parallel to the one just mined. A schematic image of a highwall mining operation is
shown in Figure 2-1.
Since a highwall mining operation only needs a narrow bench to obtain entrance to the
coal seam, highwall mining systems not only offer less environmental disturbances to the
surrounding land, but also are able to advance economic benefits substantially. The highwall
mining system is extremely mobile and can be moved from pit to pit in a few hours, or from mine
to mine in a day or two. Therefore, the efficient highwall mining systems would significantly
extend the reserve life of open-cut coal mines. Moreover, since the entries are unmanned, many
of the safety issues associated with underground mining are eliminated. In general, the highwall
mining method is very productive, flexible and cost-effective when used in proper situations.
Figure 2 - 1 A Schematic Image of a Highwall Mining Operation (Shen, et al., 2001)
2.1.2 Advantages of Highwall Mining
Highwall mining can economically access smaller blocks of coal with high flexibility and
is more readily able to avoid geological structures or other impediments to production. The
mobility of the highwall mining equipment make it easy to move around a pit, from pit to pit or
6
from mine to mine. Therefore, its applicability improves a lot contrary to other mining methods.
Due to the reason that highwall mining is operated by remote control, highwall mining machines
are able to operate in thin seams without having to cut stone to make room for people to move
around. Furthermore, it can also selectively extract a high-quality segment of a seam. In this
sense, the safety conditions will be greatly advanced. In addition, operations are inherently safer
because they are carried out by remote control, and all personnel remain outside the entries.
Personnel therefore remain on the surface to avoid hazards such as roof falls, gas, dust,
irrespirable atmospheres, flooding, vehicle movements in confined spaces, etc. Without the use of
ventilation, highwall mining can continue in gassy seam conditions that would stop or impede
underground mining.
2.2 Highwall Mining Process
The highwall mining method is a hybrid of surface and underground coal mining
methods that evolved from auger mining. A typical mining cycle includes sumping (forward
pushing) and shearing (raising and lowering of the cutter-head to extract the entire height of the
coal seam). As the cutting cycle continues, the cutter-head is progressively pushed into the coal
seam to a depth of 20 ft. Fully enclosed push-beams are inserted behind the cutter-head as it
moves forward into the coal seam. The push-beam transfer mechanism stages each section with a
push-beam above the power-head before insertion and moves the cut coal from the enclosed
push-beams to the power-head. Then, the coal drops to the conveyor system, where coal is moved
to the center of the bench for stockpiling and transport. The mining cycle continues for 20 ft and
the process is repeated until the operator reaches the desired depth. After full coal extraction, the
push-beams are removed from the back of the cutter-head. As it retracted, push-beams are stacked
in an open pit area, minimizing the on-site size requirements. Once the cutter-head has been fully
retracted, the drum is inspected and serviced. The machine is then moved to the next entry
(Fiscor, 2002). As the string of push-beams is hinged at 20-ft intervals, the machine can be
navigated vertically through rolls and undulations within the coal seam. Currently, the highwall
mining systems are capable of handling coal seam thicknesses from 2.6 to 16 ft with a dip up to
8°.
Generally, a highwall mining operation requires crew of 6 employees: two foremen, a
machine operator, a ground man, a loader operator, and a technician as needed. The operator is
located in the control cabin about 20 ft above the ground where the operator has a complete view
of all mining activities around the machine. The operator primarily controls sumping and cutting
7
operations of the continuous miner. In addition, the operator monitors six to nine screens which
provide information on methane levels, hydraulic pressure, roof and floor conditions, coal
thickness, entry heading and fellow employee activities. The ground man cleans debris from the
top of the push-beams or conveyor cars, and ensures that each push-beam is properly locked and
connected as mining progresses. The loader operator loads push-beams onto the push-beam
transfer mechanism or conveyor cars behind the continuous miner during mining. At no time do
any employees work underground (Fiscor, 2002).
2.3 Highwall Mining Systems
Currently, the highwall mining systems market are dominated by two manufacturers.
International Coal Group, Inc. (ICG), now becoming a wholly owned subsidiary of Arch Coal,
developed the ADDCAR system shown in Figure 2-2. Caterpillar developed the Cat HW300
shown in Figure 2-3, which is a continuation of the Superior Highwall Miner after purchasing
Bucyrus in 2011.
2.3.1 ADDCAR Highwall Mining System
The ADDCAR Highwall Mining system is set up in front of a highwall which is a truly
continuous system. This is due to the fact that coal production does not need to be interrupted to
add additional conveyance cars into the highwall as the miner advances to the entry. The
conveyor cars are connected together by using vertical locking pins. In this way, a continuous
haulage system among the cutting machine, the launch vehicle, and a stacker conveyor is created
(Gardner, et al., 2002).
The central component of the system is the launch vehicle, which is the central part of the
highwall mining system, acting as a platform for the essential electrical, hydraulic, and other
control functions. First of all, the launch vehicle is aligned perpendicular to the coal seam at the
preferred location, which remains stationary during the mining cycle. Then a crawler-mounted
continuous miner enters the highwall and begins to cut the coal. The 40-foot-long conveyor cars
are added, one at a time, which are placed behind the continuous miner as the system advances
under the highwall. These cars collect the coal from the continuous miner and transport it to the
outside. The conveyor cars discharge the coal car-to-car without the use of an auger, which will
not increase fines. One loading stacker conveyor receives the coal that is discharged from the
launch vehicle and loads it into awaiting trucks. When an entry has been completed, the entire
8
system will be retracted, moved over, and aligned with the next entry. (ADDCAR Highwall
Mining Systems, http://www.addcarsystems.com/).
Figure 2 - 2 ADDCAR Highwall Miner (http://www.addcarsystems.com/broad.html)
9
Figure 2 - 3 Cat HW300 Miner (https://mining.cat.com/highwall-miner)
2.3.2 Cat HW300 Highwall Mining System
The Cat HW300 highwall mining system is a highly efficient mining equipment which
offers a safe method for recovering coal from the final highwall. This system is designed with
easy maintenance and a comprehensive diagnostic system, and is able to be disassembled in
modules, which all contribute to the system’s outstanding productivity. The Cat HW300 offers
two electric cutter head modules, which are interchangeable and able to be quickly attached to the
power-head. The whole cutting cycle is fully automated, however, when the coal seam varies, it
still allows the operator to make adjustments through an ampere reading.
The self-propelled Cat HW300 Highwall miner is capable of operating on benches as
narrow as 59 ft. This highwall mining system is able to tram from entry to entry and transport
coal resources in limited space easily. The machines can also be easily moved from pit to pit or
from mine to mine. The enclosed push-beams and an anchoring system are mounted on Cat
HW300 to manage roof fall problems during mining production. As roof rock is collected on the
push-beams, the weight of the mining string increases. If too much load is added on the push
beams, the string will be retracted and the rock will be removed by the coal loader and then
reenter the entry to finish the mining cycle. The force applied to the string of push-beams must
overcome changes in elevation due to undulations and rolls as well as any roof material that may
10
fall on the top of the push-beams. Caterpillar highwall miners are equipped with 49 push-beams,
allowing mining system to penetrate nearly 1000 ft into the coal seam. (Buchsbaum, 2011;
HW300 Brochure, 2013).
2.4 Highwall Mining Guidance System
Since most of the entries fail to reach the designed or predetermined depth, it is very
important to maintain the coal pillars and keep parallel openings. However, this can be achieved
only if the highwall miners’ position and heading could be determined and controlled remotely.
Therefore, it is imperative to highlight the guidance problems associated with highwall mining,
which tend to decrease the recovery ratio as well as cause hazards to highwall stability. A variety
of sensors, keeping the miner in the seam automatically, have been used in highwall mining
operations. These sensors include a roof and floor passive gamma detector system, multiple
inclinometers, a ring laser gyroscope, and a programmable logic controller (Ralston, 2001). These
remote detection systems offer the following two purposes: (1) locating roof and floor rocks to
avoid cutting into such rock, and (2) determining the pillar width left between the previous and
current entries to maintain the pillar design width.
2.4.1 Guidance System for ADDCAR Highwall Mining System
The ADDCAR system is equipped with advanced guidance features which are able to
keep the miner operating in the seam, control pillar width as well as get maximum recovery
without risking roof stability. A combination of hydraulic, electrical and electronic technologies
is used for lateral guidance, mining a predetermined height, pushing and pulling the system into
or out of the hill and for cutting and loading the coal.
For lateral guidance system, Honeywell’s Ore Recovery and Tunneling Aid (HORTA) is
utilized in ADDCAR system. There are three pairs of ring laser gyroscopes and accelerometers,
which are used to monitor the heading, pitch and roll of the continuous miner, shown in Figure 2-
4. The operator has access to the complete picture of the active entry, past entries as well as pillar
thickness, allowing the operator to navigate the miner and make adjustments. This ability is very
important to improve the safety situations of highwall mining operations, since the correct size of
web pillars should be kept to carry overburden load.
For vertical guidance system, the thickness of the roof and floor coal is continuously
measured by gamma coal thickness sensors. With the help of these sensors, predetermined
11
thickness of roof or floor coal can be achieved. In order to ensure optimum quality and assist in
roof control, the gamma based information is used during operation. The exact cutting height with
respect to the established floor is provided with graphical data by the use of boom-mounted
inclinometers. A cable reel mounted odometer is used to measure the depth of cut. This provides
the highwall miners to achieve high penetrations and reduces out-of-seam dilution (Vandergrift,
et al., 2004; Highwall roundtable- Highwall systems, 2006).
2.4.2 Guidance System for Cat HW300 Highwall Mining System
The operation of the Cat HW300 highwall mining system is controlled through the use of
PLC (programmable logic control) technology at the heart of its machine control system. By the
use of touch screen technology, the operator in the cab can not only have immediate-response
control over the machine’s functions, but also adjust these functions in case of the unexpected
mining conditions within the entry. A comprehensive diagnostics system is constructed with the
function of troubleshooting assistance and streamlines maintenance procedures. Therefore, it is
not difficult to see that the mining operation can be optimized, resulting in cleaner coal products.
Figure 2 - 4 Lateral Guidance System of ADDCAR
(http://www.addcarsystems.com/techleader.html)
Gamma Detection systems are used to provide the machine operator with an additional
knowledge to achieve the optimum cutting range. This technology is capable of steering the cutter
module through the coal seam, recovering predetermined amounts of coal in the coal seam. This
system also allows the extraction of coal reserves in soft roof or soft floor conditions. In the past,
12
it was too easy to overcut, resulting in unwanted situations which tend to cause problems for roof
control and even worse cause hazards to mining machines. In order to achieve more accurate
direction, Caterpillar provides a navigation and steering system. (HW300 Brochure, 2013).
2.5 Multiple Seam Highwall Mining
Many highwall mining operations recover multiple seams in very close proximity with
the purpose of increasing the recovery ratio. In the eastern U.S., this situation appears frequently
when a thick seam splitting into thinner seams. In the western U.S., certain very thick seams can
exceed the working height of the highwall miner, and a multiple seam mining approach may be
utilized (Zipf, 2005). When it comes to multi-seam highwall mining, seams being mined below or
above a previously mined seam tend to be subjected to disturbances from the adjacent coal seams,
especially in close proximity. The mining induced strata movements can cause serious roof and
pillar stability problems, such as high stress concentration and heavy fracturing in the adjacent
seam. Therefore, it is imperative to prevent web pillars from collapsing and the high possibility of
highwall failure from the standpoint of ground control safety in multiple seam highwall mining
situations.
The main extraction sequence for highwall mining can be classified into two categories
based on the spatial relationship between workings on two adjacent seams: (1) under-mining
where the upper seam is mined out prior to the mining of the lower seam; (2) over-mining where
the mining of the upper seam is not commenced until the extraction in the lower seam completely
finished. The multiple seam highwall mining sequence should be carefully selected to avoid
operation concerns. For under-mining, it may lead to future web pillars collapse and subsidence
in the upper seam, leaving unstable highwall above the active pit. For over-mining, it is easier to
realize the continuity of surface mining operations and maximize the recovery ratio. This is due
to the fact that over-mining makes it easier to backfill the abandoned entries from a combination
of back stacking and push spoil into lower entries as the upper seam is developing. However, if
the thickness of interburden is very small, it may lead to mining equipment entrapment.
Moreover, because of the relatively thin web pillars, accurate surveying is required to ensure that
the adjacent highwall entries do not intersect each other. For over-mining, it is required that the
survey points be located before the operation of the highwall miner in the upper seam since the
openings are partially backfilled. However, it is much easier to stack at the beginning of an entry
if under-mining sequence is selected. This is because the upper seam is visible and positioning the
highwall miner directly below is no problem.
13
For multiple seam mining operations, it is imperative to carefully evaluate the stability
problem of highwall structures when making designs for highwall mining (Newman,
2009).Because the stability problem in multiple seams is mainly dependent on the transfer of
stress through the upper and lower web pillars, properly stacking web and barrier pillars is one of
the very important solutions to prevent highwall failure from occurring. If a highwall miner cut is
not aligned properly with the overlying or underlying cut, the overburden stress is transferred
onto the interburden. Under this situation, failure is likely to occur as web pillars punch into the
interburden, which may cause great danger to highwall mining operations. Due to web pillars
being very narrow and extremely long, it is very difficult to columnize web pillars between the
upper and lower seams. However, it is much easier to maintain barrier pillars columnized because
of their greater width. Moreover, barrier pillars are capable of supporting the load transferred
from web pillars, thereby stiffening the highwall system, and thus preventing the catastrophic
collapse. Therefore, it is important to design barrier pillars with a proper size because they must
have adequate strength to withstand additional stresses imposed on them if all web pillars failed
within a panel. In addition, in order to maintain the stacking deep within the entries, an on-board
guidance system should be used. Besides properly stacked web and barrier pillars, limiting the
number of entries will also lessen the possibility of highwall failure in close proximity multiple
seam highwall mining situations.
2.6 Empirical Highwall Mining System Design
Highwall mining should follow a detailed plan to control the hazards which may happen
during highwall mining operations. When designing a highwall mining layout, the mining
designer must determine 1) web pillar width, 2) the number of web pillars between barrier pillars
and 3) the barrier pillar width. The design parameters are determined by the highwall miner entry
width, the mining height and the overburden depth (Zipf, 2005). In addition, the pillar strength,
the applied stress on pillars, and the pillar stability factor should be estimated by the mine
designer.
2.6.1 Coal Pillar Strength
The Mark-Bieniawski formula applies best for web pillars, which are very long, narrow
rectangular pillars. For long pillars whose strength length is much greater than their width, the
Mark-Bieniawski formula can be reduced to the Equation 2-1. In-situ coal strength is normally
selected as 900 psi. Mining height depends on the seam thickness.
14
𝑆𝑝 = 𝑆𝑖 ∙ 0.64 +0.54𝑊
𝐻 (2 − 1)
Where: 𝑆𝑝–web or barrier pillar strength
W– web or barrier pillar width
𝑆𝑖 –in-situ coal strength
H–mining height
2.6.2 Coal Pillar Stress
For the web pillar system, the tributary area method is used for calculating the coal seam
vertical stress on web and barrier pillars. Average vertical stress on a web pillar is shown in
Equation 2-2. The highwall mining equipment dictates the entry width which is either 9.5 or 11.5
ft. In-situ vertical stress is determined by two factors, namely overburden depth and the overlying
rock density. Vertical stress gradient is typically 1.1psi/ft. Overburden depth may be selected as
the maximum overburden depth on a highwall mining web pillar, which is very conservative and
can be computed using a high average value shown in Equation 2-3. Finally, the stability factor
for a web pillar is shown in Equation 2-4. For design purposes, the stability factor for a web pillar
typically ranges from 1.3 to 1.6.
𝜍𝑤𝑝 = 𝜍𝑖 ∙𝑊𝑤𝑝 + 𝑊𝑒
𝑊𝑤𝑝 (2 − 2)
𝐷𝐷𝑒𝑠𝑖𝑔𝑛 = 0.75 × 𝐷𝑚𝑎𝑥 + 0.25 × 𝐷𝑚𝑖𝑛 (2 − 3)
𝑆𝐹𝑤𝑝 =𝑆𝑤𝑝
𝜍𝑤𝑝 (2 − 4)
Where: 𝑊𝑒 –highwall miner entry width
W𝑤𝑝 –web pillar width
𝜍𝑖 – in-situ vertical stress
𝐷𝑚𝑎𝑥 – maximum overburden depth
𝐷𝑚𝑖𝑛 –minimum overburden depth
15
For the barrier pillar system, when the number of web pillars in a panel is N, the panel
width can be computed by Equation 2-5. The average vertical stress on a barrier pillar can be
determined by Equation 2-6. Similarly, the stability factor for a barrier pillar is shown in Equation
2-7. Since the stress undertaken by web pillars is neglected, the stability factor for a barrier pillar
can be as low as 1.
𝑊𝑝 = 𝑁 𝑊𝑤𝑝 + 𝑊𝑒 + 𝑊𝑒 (2 − 5)
𝜍𝑏𝑝 = 𝜍𝑖 ∙𝑊𝑃 + 𝑊𝑏𝑝
𝑊𝑏𝑝 (2 − 6)
𝑆𝐹𝑏𝑝 =𝑆𝑏𝑝
𝜍𝑏𝑝 (2 − 7)
Where: 𝑊𝑃– panel width
𝑊𝑏𝑝–barrier pillar width
2.7 Introduction of Examine2D and FLAC
2.7.1 Methods of Numerical Modeling
Numerical modeling could be used to design and evaluate mine structures and support
systems. The advantage of numerical modeling is that model construction is much easier and
quicker than the physical modeling, and the design parameters can be changed easily to evaluate
the effect of parameters’ change on the overall design. The most commonly applied numerical
methods for rock mechanics problems are (Jing, 2002):
(1) Continuum methods including: the finite difference method (FDM), the finite element
method (FEM), and the boundary element method (BEM)
(2) Discrete methods including: the discrete element method (DEM) and the discrete
fracture network method (DFN)
For finite element method (FEM), the basic principal is that a solution region can be
analytically or approximately modeled by replacing it with an assemblage of discrete elements.
Because these elements can be permutated in a variety of ways, they can be used to represent
exceedingly complex shapes. For finite difference method (FDM), numerical techniques are used
16
to approximate the solutions of differential equations by replacing the partial derivatives with
finite difference approximations defined at neighboring grid points. The advantage for FEM and
FDM is that these methods allow material to deform and fail, and thus these methods are capable
of modeling complex behavior. And simple structures can be simulated with interfaces, but not
suitable for highly jointed-blocky media. Also, there are some limitations should be noticed
including effects of mesh size, boundaries, symmetry restrictions and data input limitations (such
as effects of variation of critical input parameters). In the boundary element method (BEM), the
governing differential equations are transformed into integral variables, which can be used over
the boundary surface of the region. The advantages are able to do elastic and rapid assessment of
designs and stress concentrations (Peng, 2008).
In the discrete element methods, the blocks are in mutual contact and the contact is
represented by springs in both normal and tangential directions. Due to the fact that natural
characteristics of rock mass are comprised of blocks bounded by joints, there is a large
applicability in the field of geomechanics. However, the drawback is that it is difficult to obtain
reliable data on location, orientation and persistence of the discontinuities. For discrete fracture
network (DFN), it is most suitable for the study of fluid flow and mass transport in fractured
rocks for which an equivalent continuum model is difficult to model or establish (Jing, 2003).
2.7.2 Introduction of Examine2D
Examine 2D is designed to be a quick and simple to use parametric analysis tool for
investigating influence of geometry and in-situ stress variability on the stress changes in rock due
to excavations. The induced stresses can be analyzed by means of stress contour patterns in the
region surrounding the excavations. As a tool for interpreting the amount of deviating overstress
(principal stress difference) around openings, strength factor contours give a quantitative measure
of strength over induced stress according to a user defined failure criterion for the rock mass
(Rocscience, 2012).
Some important limitations of the program should be considered when interpreting
Examine2D output. First of all, the program is based on the assumption of plane strain, which
means that the modeled excavation is of infinite length normal to the plane section of the
analysis. In practice, the out-of-plane excavation length should be larger than five times the
largest cross-sectional dimension. Second, the material being modeled in the Examine2D is
assumed to be homogenous, isotropic or transversely isotropic, and linearly elastic. Third, the
17
displacements shown by Examine2D are meant to qualitatively illustrate regional deformation
trends only. In general, the induced stresses calculated and displaced by Examine2D can usually
prove useful and may still yield useful insight in the analysis.
2.7.3 Introduction of FLAC
FLAC is a two dimensional finite difference program for engineering mechanics
computation, which is capable of simulating the behavior of structures built of soil, rock or other
materials. In FLAC, materials are represented by elements or zones, which is able to form a grid
to fit the shape of the object to be modeled. With the purpose of responding to the applied forces
or boundary restrains, these elements would behave by following the prescribed linear or
nonlinear stress/strain law. FLAC also consists of built-in programming language FISH. Because
of FISH, the user can write functions to extend FLAC’s usefulness. At the same time, FLAC can
be operated as either a menu-driven or a command-driven computer program (Itasca, 2011).
With the purpose of setting up a model with FLAC, three fundamental components of a
problem must be specified: (1) a finite difference grid; (2) constitutive behavior and material
properties; and (3) boundary and initial conditions. Among them, the grid defines the geometry of
the problem, the constitutive behavior and associated material properties dictate the type of
response the model will display, and boundary and initial conditions define the in-situ state. In
FLAC, the time-marching method is adopted to solve the algebraic equations and the solutions
are reached after a series of computation steps. In the meantime, the number of steps required to
reach a solution can be controlled by code or manually by the user. The generation solution
procedure is illustrated in Figure 2-5, which represents the sequences of processes that occurs in
the physical environment.
In FLAC, there are twelve built-in constitutive material models: null model; elastic,
isotropic model; elastic, orthotropic model; elastic, transversely isotropic model; Drucker-Prager
model; Mohr-Coulomb model; ubiquitous-joint model; strain-hardening/softening model; bilinear
strain-hardening/softening ubiquitous-joint model; double-yield model; modified Cam-clay
model and Hoek-Brown model. In this thesis, the Mohr-Coulomb material is used to simulate the
real behavior of the highwall structure.
18
Figure 2 - 5 General FLAC Solution Procedure (Itasca, 2011)
19
2.7.4 Mohr-Coulomb Model
Mohr-Coulomb Model is used in this model construction to simulate the real behavior of
highwall structures. The reason to choose this model is that the Mohr-Coulomb failure criterion is
frequently used to assess the state of failure in the study of soil and rock mechanics. However, the
drawback is that the effect of intermediate principal stress will not be taken into consideration. In
FLAC, the material properties that must be defined for a Mohr-Coulomb material include:
density, bulk modulus, shear modulus, friction angle, cohesion, dilation angle, and tensile
strength.
The failure envelop for this model corresponds to a Mohr-Coulomb criterion (shear yield
function) with tension cutoff (tension yield function). The tensile flow rule is associated and the
shear flow rule is non-associated. In the FLAC model, principal stresses 𝜍1 ,𝜍2 ,𝜍3 are used, the
out-of-plane stress, 𝜍𝑧𝑧 , being recognized as one of these. The principal stresses and principal
directions are evaluated from the stresses’ tensor components and ordered so that𝜍1 ≪ 𝜍2 ≪ 𝜍3.
The corresponding principal strain increments ∆𝑒1 ,∆𝑒2 ,∆𝑒3 are decomposed as follows:
∆𝑒𝑖 = ∆𝑒𝑖𝑒 + ∆𝑒𝑖
𝑝𝑖 = 1,3 (2 − 8)
Where the superscripts e and p refer to elastic and plastic parts, respectively, and the
plastic components are nonzero only during plastic flow.
With the ordering convention of equation 2-8, the failure criterion may be signified in the
plane (𝜍1 ,𝜍3) as illustrated in Figure 2-6.
The failure envelop is defined from point A to point B by the Mohr-Coulomb yield
function 2-9:
𝑓𝑠 = 𝜍1 − 𝜍3𝑁∅ + 2𝑐 𝑁∅ (2 − 9)
From B to C by a tension yield function 2-10:
𝑓𝑡 = 𝜍𝑡 − 𝜍3 (2 − 10)
Where ∅ is the friction angle, c, the cohesion, 𝜍𝑡 , the tensile strength is 𝑁∅.
𝑁∅ =1 + 𝑠𝑖𝑛∅
1 − 𝑠𝑖𝑛∅ (2 − 11)
20
Figure 2 - 6 Mohr-Coulomb Failure Criterion in FLAC (Itasca, 2011)
In FLAC, only the major and minor principal stresses are effective in the shear yield
formulation, which means the intermediate principal stress has no effect. For a material with
friction, ∅ ≠ 0 and the tensile strength of the material cannot surpass the value 𝜍𝑚𝑎𝑥𝑡 given by
equation 2-12.
𝜍𝑚𝑎𝑥𝑡 =
𝑐
𝑡𝑎𝑛∅ (2 − 12)
21
Chapter 3 Application of the Highwall Mining Method
3.1 Suitability of Highwall Mining in West Virginia
There are several mining methods that are popularly used in recovering coal seams in
West Virginia, such as underground mining, mountain-top-removal mining, contour mining,
auger mining, and highwall mining. Many factors should be taken into consideration when
selecting the most suitable mining method from above list. The most significant factors should be
taken into account are geology and topography. Most of the high-quality and high-value coal in
the state of West Virginia is produced in the mountainous areas of the central Appalachian
coalfields. In most cases, the multiple coal seams are formed in close proximity as shown in
Figure 3-1. From this figure, it is easy to find that some of the coal seams recovered by the
mountain-top-removal method would often result in too much damage to the surface plants,
disturbance of streams, and movement of overburden rock and surface soil.
Figure 3 - 1 Geological Column at a Southern WV Mine Site (Kitts, 2009)
22
3.1.1 Requirements for Underground Mining
If coal seams are thick enough (e.g., >3ft in height) that could be recovered by
underground mining methods, the suitability of adopting underground mining methods is
restricted by coal seams’ width between the outcrops on both mountain sides as shown in Figure
3-2. A minimum width is required to accommodate two outcrop barrier pillars on the mountain
sides, and a 5-entry mains system is placed between the outcrop barriers (Kitts, 2009).
Based on underground mining regulations, it is required that overburden depth should be
larger than 100 ft within the inner edge of an outcrop barrier. Therefore, Equation 3-1 can be
applied to determine the minimum coal seam width. However, in order to ensure a profitable and
efficient mining operation, the coal seam width should be much larger than the minimum width
derived from Equation 3-1 (Luo, 2014).
𝑊𝑚𝑖𝑛 = 2 ×100
𝑡𝑎𝑛𝛼+ 𝑁𝑊𝑒 + 𝑁 − 1 𝑊𝑝 (3 − 1)
Where: 𝑊𝑚𝑖𝑛 – minimum width of the coal seam between the outcrops
𝛼–angle of the mountain slope
N –number of entries required in the mains
𝑊𝑒–mine entry width, typically 20 ft
𝑊𝑝–rib-to-rib width of the pillars in the mains system
For example, in order to construct an underground coal mine under a ridge top with a
typical surface slope of 30°, 30 ft pillar width, and20 ft entry width, the minimum coal seam
width should be about 570 ft. Only a small portion of the coal reserves in the example in the
Figure 3-2 can be recovered using an underground mining method.
23
Figure 3 - 2 Typical Appalachian Ridge Top Coal Reserve (Kitts, 2009)
3.1.2 Limitations of Contour Mining
The contour mining method can only recover a restricted amount of the coal reserves
from the outcrop location. With the purpose of extracting more coal from a coal seam, it is
required to increase the width of the contour bench. However, the amount of overburden to be
removed for creating the coal bench and the increase of production cost are in proportion to the
square of the bench width. Therefore, the market price of the coal and economics of production
cost would determine the bench width. The maximum bench width can be yielded by carrying out
an economic zero profit analysis between the value of the coal mined and the cost to remove the
overburden. The dollar per cubic yard is usually used to present the unit cost to remove the
overburden strata. The total volume of overburden to be removed for exposing the contour bench
width of 𝑊𝑏 and one-foot development along the longitudinal direction of the contour bench
could be derived from the equation shown in Equation 3-2 (Luo, 2014).
24
Figure 3 - 3 Contour Mine Bench Design
V =1
54𝑊𝑏
2 𝑠𝑖𝑛𝛼 × cos(𝛽 − 90)
sin(180 − 𝛼 − 𝛽)𝑦𝑑3 (3 − 2)
Where: V–volume of overburden rock to be removed, yd3
𝑊𝑏– bench width, ft
𝛼–slope angle, degrees
𝛽–highwall angle, degrees
The maximum bench width based on an economic break-even analysis for a contour
mining operation can be determined by:
𝑊𝑏 ,𝑚𝑎𝑥 = 0.027 ×𝐶𝑐𝑜𝑎𝑙
𝐶𝑂𝑅×
𝑚 ∙ 𝛾 ∙ 𝜂 ∙ 𝑠𝑖𝑛(180 − 𝛼 − 𝛽)
𝑠𝑖𝑛𝛼 ∙ cos(𝛽 − 90)𝑓𝑡 3 − 3
Where: 𝑊𝑏 ,𝑚𝑎𝑥 –the maximum bench width, ft
𝐶𝑐𝑜𝑎𝑙 –coal price, $/ton
𝐶𝑂𝑅–unit cost of overburden removal, $/𝑦𝑑3
m –coal thickness, ft
𝛾–coal density, lbs/𝑓𝑡3
25
𝜂–coal wash recovery rate
For example, a contour mining operation has the following design and operating
parameters:
Surface slope: 30°
Highwall angle: 95°
Coal thickness: 36 inches
Coal wash recovery rate: 95%
Coal density: 85lbs/𝑓𝑡3
Overburden removal cost: $3/𝑦𝑑3
Coal price: $40/ton
Using Equation 3-3, the maximum bench width derived from the break-even analysis
should be smaller than 143.4ft. Therefore, a significant volume of coal reserves could be left
unmined if only the contour mining method is applied to recover those coal seams presented in
the Figure 3-2 case.
3.1.3 Auger Mining
Prior to the development of the highwall mining system, the auger mining method is
widely used to recover the coal resources left by contour mining. As the cutter heads cut through
the coal seam, auger drills and flights are inserted behind the cutter head. The cutter head on the
auger excavates a number of entries into the seam, which operates very similarly to how a wood
drill produces wood shavings. The coal is then recovered and moved back to the surface through
the spiral action of flights. However, as the depth of the bored entry is extended, the drilling
power is diminishing and the coal production is decreased. When the auger reaches the maximum
torque, the maximum length of a highwall entry is determined. There are also some other
disadvantages associated with augers that restricted the use of auger mining. Due to the fixed
cutting height of auger mining, it tends to reduce the recovery ratio of the coal resources
significantly, which only recovers 30% to 40% of the coal within that coal seam to that depth. It
also has no ability to negotiate dips and rolls in the coal seam because of the rigid structure of the
auger flights.(http://www.ritchiewiki.com/wiki/index.php/Auger_Mining#ixzz3RM9tPgum)
26
3.1.4 Suitability of Highwall Mining
Because of the special geological and topographical structures in the Appalachian
coalfields, especially in central Appalachia region, all four of the popular coal mining methods
(i.e., mountain-top-removal, underground, auger and contour) have their restrictions to recover
some of the high-quality and high-value coal seams in this area. However, as for highwall mining,
many of the unsatisfactory aspects of other coal mining methods have been improved. With its
capacity to penetrate into the coal seams for a great distance with almost constant power, it can
extract a large amount of the coal reserves. With a highwall mining system equipped with
adjustable continuous miner cutting head, it can recover variable mining heights which increases
the recovery of these coal reserves to a great extent. Simultaneously, no coal size is necessarily
degraded with the increasing mining depth. Since there is no rigid structure of the push beams,
the dips and rolls can be negotiated in the coal seam, which is capable of advancing the
production rate greatly. Although highwall mining is a hybrid of surface and underground mining
technologies, it is much safer compared to the underground mining method because no in-seam
support or transport systems are required, and ventilation measures are negligible. In fact, the
highwall mining method combined with contour mining would became the most preferred mining
method for recovering many of the coal seams exposed on the mountainsides in the Appalachian
coalfields.
3.2 Main Challenges with Highwall Mining
3.2.1 Types of Highwall
According to Gardner and Wu (2002), there are three types of highwalls in highwall
mining, namely unreclaimed highwalls, surface mining highwalls, and highwall mining
highwalls. Unreclaimed highwalls are abandoned highwalls from previous contour mining,
including auger mining. In this case, the highwalls are most likely not appropriately tilted and the
highwalls are not stable. Surface mining highwalls are operated on the final bench of surface
mining operation. Highwall mining highwalls are the most favorable ones due to the fact that they
are usually well located and designed to enhance stability and productivity. Among the three
types of highwalls, unreclaimed highwalls demand the most attention. This type of highwall may
have been degraded due to weathering, ground water flow, and failure of web pillars. Therefore,
mining operators should examine this type of highwall very carefully and take methods to avoid
any potential failure of the highwall before putting them into use.
27
3.2.2 MSHA Incident Statistics for Highwall Mining
Highwall mining is essentially a hybrid of surface and underground mining technologies.
Although all highwall mining workers stay in the surface, the highwall mining production is still
operated in the underground environment. Therefore, highwall mining is capable of resulting in
the stress redistribution in the overburden strata, which poses potential hazards to mine workers
as well as highwall mining machines.
Based on the distribution of fatalities classified by the Mine Safety and Health
Administration (MSHA), ground control, particularly the highwall stability, is the principal
source of highwall accidents. Powered haulage and machinery accidents are other prominent
sources (Zipf, 2005). At most times, slope stability accidents and highwall failures not only pose
serious risks to coal mine workers, but also result in at least a week off from work to recover
mining machines and resume highwall operations. In most cases, such accidents occurred because
mine management failed to recognize a geologic abnormality and failed to adopt rational mining
design to ensure highwall stability. In order to prevent the accidents happening, MSHA has
proposed some best practices in the following (MSHA, 2015):
(a) Train all highwall workers to recognize the highwall hazards.
(b) Highwall needs to be inspected before, during, and after every rain, freeze, or thaw.
Examine the face of the highwall, benches, and areas with cracks and loose rocks.
Sloughing over hangs, ground, and large rocks that could cause potential safety hazards
to highwall mining operations.
(c) Pay attention to loose highwall material and never work under them. Cut down loose
precarious material on a safe position. When dangerous situations fail to be modified,
barricade and post signs to stop entering the working area. Be cautious to the loose
highwall material especially when working close to the corners of highwalls. Points and
outside corners of highwalls are naturally unstable due to weathering and erosion and
toppling failures are likely to occur under this geological situation.
(d) Specify adequate size of the highwall benches in the highwall mine design.
(e) Clear all the gatherings of fallen rock from benches before conducting operations in
the protected area.
28
(f) Convey dangerous situations to other workers and equipment operators. Notify them
of hazardous highwall situations with the help of radios or cell phones.
3.2.3 Types of Highwall Failures
Failure of highwall and highwall falls have been the most serious safety hazards in
highwall mining operations. In general, a highwall failure is the unintended loss of material from
a highwall. There are many factors that contribute to highwall instability, including rock mass
properties, highwall geometry, face orientation, precipitation, groundwater, freeze, equipment
vibrations, blasting, and so on. On the whole, there are two types of highwall failures, namely
rock mass failures and rock falls.
3.2.3.1 Rock Mass Failure
Rock mass failures generally involve a relatively large amount of material on a large
portion of a highwall and it is important to make sure the material or structures are controlled.
There are four types of rock mass failure, namely planar, wedge, toppling, and circular. Toppling
is the most common failure mode, circular and wedge follow, and the possibility for the planar
failure to occur is the lowest (MSHA, 2014).
3.2.3.1.1 Planar Failures
The planar failure mode refers to a situation in which the sliding movement occurs along
a single discontinuity surface, shown in Figure 3-4. Planar failures require an adverse potential
failure plane striking subparallel to the face and release surfaces at the top and both ends (Bullock
et al., 1993). In general, a tension crack would appear on the upper slope surface and the failed
block would detach from the slope and slide down along the plane of failure when planar failure
occurred. When heavy rains occur, water tends to flow into the tension crack and the friction
resistance is reduced to a great extent, which also accounts for the main factor causing planar
failure. Therefore, the location and severity of the tension crack deserves more attention in order
to prevent this type of failure.
29
Figure 3 - 4 Planar Type of Highwall Failure (MSHA, 2014)
3.2.3.1.2 Wedge Failure
The wedge failure occurs when sliding movement along two involved discontinuity
surfaces that intersect at an angle forming a wedge shaped block in the highwall face, shown in
Figure 3-5. Typically, a bedding plane forms on the upper surface of the wedge and sliding occurs
along the intersection or on one of the two discontinuity surfaces. There are many factors that can
trigger the wedge type of highwall failure, such as mining activities or water flow, which are
capable of reducing the friction resistance of the potential failure planes.
3.2.3.1.3Toppling Failures
The toppling failure mode refers to a situation in which bulking or rotational movement
occurs around the base of a slab or column, shown in Figure 3-6. In a highwall, due to the fact
that the stress component normal to the highwall face does not exist, the highwall tends to expand
toward the free face and splits off parallel or subparallel to the highwall face.
30
Figure 3 - 5 Wedge Type of Highwall Failure (MSHA, 2014)
Figure 3 - 6 Toppling Type of Highwall Failure (MSHA, 2014)
31
3.2.3.1.4Circular Failures
In general, rotational and sling movement along a failure surface are involved in circular
failures, shown in Figure 3-7. This type of failure usually occurs along numerous discontinuities
and the shape of this type of failure is often like the arc of a circle. Typically, circular failures are
the least favorable type of failures, which may extend from the crest to the toe of the slope.
Figure 3 - 7 Circular Type of Highwall Failure (MSHA, 2014)
3.2.3.2 Rock Falls
Rock falls are a type of failure where intact blocks of rock on the fragmented highwall
fall down because they are unconfined. There are several critical factors in evaluating the rock
fall hazards, namely exposure, block weight, drop height, and highwall geometry. Among these
factors, block weight and drop height play an important role in determining the extent of damage
of a falling rock, and geometry of the highwall will affect how a rock falls and where it lands
(MSHA, 2014).
3.2.3.3 Highwall Failure Examples
One highwall mining accident example occurred in Martin Country, Kentucky on May
24th, 2000 (MSHA, 2000). One front-end loader operator was fatally injured by the collapse of
the highwall (Figure3-8) when he was moving coal from the stockpile in the highwall miner. An
32
unexpected fall of a larger vertical plate of overburden strata was involved in this accident,
roughly 12 ft in depth and up to 264 ft in length. The fall of the plate was generated by the
extensive failure of the mine pillars under the highwall. Apparently, the primary reason of this
accident was the inadequate loading capacity of the highwall pillars for the extreme load of
detached rock plate.
Figure 3 - 8 Photo and Site Map of a Highwall Collapse in a Highwall Mining Operation
(MSHA, 2000)
33
Figure 3-9 shows that one highwall fall accident occurred leading to serious damage to
the highwall mining machines. Again, the fall occurred along the nearly right angle rock joint
interface. The kind of failure is most likely generated because of insufficient loading capability of
the web and barrier pillars at the mouth section of the highwall entries.
Figure 3 - 9 Highwall Fall along Joint Interface in a Highwall Mining Operation
34
Figure 3-10 shows the highwall mining machines are damaged by the falling rocks from
the top soil and weathered rock zone. It is quite easy to find that the highwall structures under the
failed top zone has become instable first.
Figure 3 - 10 Highwall Failure Cases in Highwall Mining Operations
3.2.4 Factors Affecting Highwall Stability
Overall, highwall mining seems to be a very safe mining method, based on the analysis of
MSHA accident and injury statistics. Since highwall mining incorporates elements of both
surface as well as underground mining, it can avoid many of the safety and stability concerns
which surface or underground mining may encounter during operation. However, some unique
safety concerns associated with highwall mining need to be addressed. Generally, the factors
affecting highwall stability during highwall mining operations can be summarized into two
categories, namely geologic structure and highwall structure stability (Zipf, 2005).
35
3.2.4.1 Geologic Structure
The principal geologic structures affecting highwall stability in the Appalachians
coalfields are hillseams. Hillseams are almost perpendicular to the bedding planes and provide
ground water channels to permeate downward from the surface, and correspondingly the
weathering process is accelerated in the fracture walls (Sames, G. P. &Moebs, N. N., 1989).
Hillseams commonly contain a joint with right angles or closely spaced joints that are weathered,
as indicated by mud or softening of the neighboring rock. They extend several hundred feet down
from the surface and their orientation is nearly parallel to the hillside on the whole. In general, a
secondary set of vertical fractures to the main fracture may exist with these hillseams. Hillseams
may give rise to long rectangular slabs or vertical wedges to separate from the highwall. There
may exist a highwall stability safety hazard leading to large rock falls from the highwall when
rock slabs that form along the hillseams detach and fall away from the highwall face. For the sake
of avoiding large rock falling from highwall, many highwall mining operators would choose to
skip an entry where a hillseam enters the highwall. A highwall containing hillseams example is
shown in Figure 3-11 (Zipf, 2005).
Unfortunately, it is out of the question to control the locations of hillseams and detecting
their presence within a highwall is not reliable. However, a number of measures can be taken to
reduce the potential failure associated with hillseams. First, those areas of the highwall where a
prominent hillseam enters highwall can be skipped when planning. In this way, the layout of
highwall mining panels is adjusted with the purpose of locating barrier pillars away from the
unstable areas. Second, it is essential to inspect and monitor the benches above the active
highwall mining area every day. Third, it is practical to reduce the slope angle of highwall from
90° to 70°- 80°, which is favorable in reducing the hillseam hazards. Some other methods such as
performing a good blasting practice are also very helpful in decreasing the damage to the
highwall structures.
The discussions above make clear the importance of performing geologic mapping to
identify geologic structures that may be naturally occurring or induced by blasting or stress-
relaxation during highwall mining operations. The orientation and extent of these geologic
defects should be mapped in advance. Attention should be paid to identifying hazards, which can
give the operator some suggestions to the expected conditions and avoid some potential hazards
such as equipment entrapment. Since groundwater pressure has the function of destabilizing a
highwall and accelerating weathering, seepage should be effectively controlled. It is quite
36
important to control the water rush into the highwall face on rainy days in highwall mining.
Moreover, some minerals that are often associated with coal such as shale, siltstone, and
mudstone show an excessive slaking behavior when they come into contact with water, leading to
a severe deterioration of their properties (Matsui, et al., 2004). Therefore, particular attention
should be given to mine water problems that have a great chance of interfering with extraction
work and decreasing the stability of highwall opening.
Figure 3 - 11 Hillseams Indicated by Arrows in Contour Mine Highwall. (Note That
Weathering along Hillseam Can Extend Several Hundred Feet or More below the Surface
Zipf, 2009)
3.2.4.2 Highwall Structure Stability
Another significant concern related to highwall mining is the stability of highwall
structure. Because no artificial roof support is installed during the entire highwall mining
operation (unless excavated entries are backfilled), the ground must be designed to be self-
supporting. Large roof falls in a highwall entry could generate substantial stability problems to a
highwall mining operation. It usually takes a long time to recover the entrapped equipment and
37
then as a result, the mining operation has to be interrupted. Pillars are left between the highwall
entries to support the overburden during and after highwall mining. If pillars collapse during the
highwall mining operation, it is quite likely for the overburden to cave into the entries, resulting
in equipment entrapment as well as significant loss of mineable resources. Even worse, if one
pillar fails the neighboring pillars are immediately overloaded, causing them to fail and so forth
until all pillars in the layout area have failed. This domino-type failure posts a great hazard to all
highwall miners (Zipf, 1999). Since a highwall mining system is extremely expensive, any kinds
of failure of highwall structures that could make the highwall mining equipment entrapped would
lead to a significant economic loss to the mining company. In addition, it is quite dangerous to
recover the entrapped highwall mining equipment underground. Therefore, design and
operational efforts should emphasize maintaining the stability of the highwall structures for the
sake of a successful highwall mining operation (Shen, 2001).
Figure 3 - 12 Highwall Collapse in Multiple Seam Mining Area (Zipf, 2009)
It is fairly common to conduct highwall mining operations in multiple seams, especially
in the central Appalachian coalfields. However, serious ground control problems could affect
highwall mining operations when encountering multiple coal seams in very close proximity,
especially in the situation that the thickness of interburden is less than the width of the highwall
entry (Zipf, 2009). Figure 3-12 shows one example where highwall mining was conducted in two
close coal seams. The upper seam was 3ft thick and the lower seam was 3 ft thick. The two seams
38
were separated by a weak, laminated interburden with a thickness from 4 to 10 ft. The extensive
highwall failure has occurred due to the domino failure of the web pillars.
Therefore, proper layout of these web and barrier pillars has a significant effect on coal
resources recovery and is very essential for highwall stability as well. The highwall structures
could be destabilized by the failure of web pillars and the following subsidence of overburden
strata. Collapse of pillars can also lead to entrapment of highwall mining machines. It is quite
dangerous to recover the entrapped highwall mining equipment. Because of these reasons, it can
be established that pillar design is the key factor to highwall mining layout. An over-optimal
design may pose too much risk for mining personnel and equipment, while over preservative
design causes unnecessary loss of resources.
39
Chapter 4 Design Methods for Highwall Mining Operations
Although highwall miners have been widely used to recover the coal resources from
previous mining operations, the methods that can be used to systematically design a highwall
mine and satisfy the requirement of safety and operational challenges are still evolving. Highwall
mine structure design methods will be introduced in this section that are critical for the safety of
miners and mining operations. The design methods and the stability analysis methods for
highwall mining operations are applicable to both single coal seam and closely spaced multiple
coal seams, respectively. With the purpose of achieving high recovery ratio, and avoiding
highwall mine structure failure as well as surface subsidence, an optimization design process is
established as well.
4.1 Stability of the Highwall Top Surface
It is not difficult to find apparent landslide of the intact blocks of rock on the top of the
fragmented highwalls in the middle part of Figure 3-10. Due to the relatively large elevation
difference between the top and bottom of the highwall, any sliding of the top soil and debris from
the top surface of the highwall will put the workers and mining equipment on the working bench
at risk. However, since a highwall mining operation normally starts from the foot of an open-pit
mine or from the previous contour mine benches, the stability of the highwall top surface should
have been dealt with before the highwall mining operations. Even in such a case, the highwall
mining operators should examine the top surface of the highwall very carefully and take
precautionary actions to avoid any potential land sliding at the highwall top.
4.2 Stability of the Highwall
Due to the close distances between the highwall and miners, and the highwall and surface
equipment, the stability of the highwall itself is a potential safety threat for a highwall mining
operation as well. Just like the stability of the highwall top surface, the highwall is generally
stable. This is because highwalls are created with the rational design during the contour and open-
pit mining operations. However, when sufficient movements and deformations occur at the
bottom of the highwall due to highwall mining operations, it is quite likely for the highwall itself
to become unstable. Since the fundamental cause for such a highwall instability problem is the
instability of pillars at the mouth section, rational design of the pillar system at the entry mouth
40
section is the primary method to stop the highwall failure. Maintaining highwall stability through
pillar design will be discussed later.
4.3 Stability of the Entry Roof
Large roof falls in a mine entry could generate substantial stability problems to a
highwall mining operation. It usually takes a long time to recover the entrapped equipment and
then as a result, the mining operation has to be interrupted. It is quite likely for roof falls to occur
when encountering weak and thinly bedded immediate roof strata. There are generally two forms
of roof falls: (1) tensile failure at the middle part of the mine entry due to excessive roof sag and
bed separations, and (2) cutter roofs at the corner of the entry. It should be noted that the width of
a mine entry created by a continuous miner in a highwall mining operation is either 9.5 or 11.5 ft
(2.9 or 3.5 m) wide depending on the highwall miner being used, which is much smaller than the
regular width of the mine entries and crosscuts in underground coal mines. However, since no
artificial roof support is installed during the entire highwall mining operation, the bed separation
from the overlying layers and sagging of thin and weak rock layer in the immediate roof should
be paid great attention. In order to assess the stability of the mine roof, a rock layer can be treated
as a beam with fixed ends once it detaches from its overlying strata as shown in Figure4-1.
Figure 4 - 1 A Beam with Fixed Ends for Assessing Mine Roof Stability in Highwall Mining
Using the beam theory, the deflection of the detached roof layer at a given point of
interest can be determined by using the following equation:
𝑆 𝑥 =𝑤𝑥2
24𝐸𝐼(𝑊𝑒 − 𝑥)2 (4 − 1)
41
Where: 𝑆 𝑥 – beam deflection at the point of interest, inches
𝑥 – distance from the pillar edge, inches
w– load per unit length of a 1-inch thick beam ( in entry axial direction), lbs/in
𝑤 =𝑏𝜌
1728
b– layer thickness, inches
𝜌 – density of the rock, lbs/ft3
E – modulus of elasticity, psi
I – moment of inertia, inch4
𝐼 =𝑏3
12
𝑊𝑒 – width of the entry, inches
Through combining the moment of inertia and load w into the equation, the roof
deflection can be expressed by Equation 4-2a. The maximum deflection at the entry center (Smax)
is shown in Equation4-2b and it can be used as an indicator for roof stability. Once the layer sags
significantly, it could increase the likelihood of roof failure considerably. From Equation 4-2b, it
is not difficult to note that the maximum deflection of the sagging layer is proportional to the rock
density as well as the entry width, and inversely proportional to the elastic modulus and the layer
thickness. Especially, the entry width and the thickness of the rock layer will play an important
role in determining the maximum deflection. This means that if the rock layer is relatively strong
and the layer thickness is relatively large, the chance for this type of bed separation from the
overlying layers to happen is relatively small. The roof deflection at a given point can also be
expressed in the term of the maximum roof deflection (Smax) in Equation4-2c.
𝑆 𝑥 =𝜌𝑥2
3,456𝐸𝑏2(𝑊𝑒 − 𝑥)2 (4 − 2𝑎)
𝑆𝑚𝑎𝑥 =𝜌𝑊𝑒
4
55,296𝐸𝑏2 (4 − 2𝑏)
𝑆 𝑥 =16𝑆𝑚𝑎𝑥 𝑥2
𝑊𝑒4 (𝑊𝑒 − 𝑥)2 (4 − 2𝑐)
42
If the immediate roof is a soft rock layer, the thickness of the rock layer above a pillar
will be shrunk by the additional vertical compressive stress after the entries adjacent to the web
pillar are mined. For a web pillar, before extraction, the load supported by a web pillar is equal to
the roof area of the web pillar. However, after extraction, the load supported by a web pillar
equals the total area of the entry and the web pillar. The incremental load, originally carried by
the entry, will also be undertaken by the web pillar. Therefore, the vertical compressive strain in
the roof layer at that time can be determined by Equation4-3.
∆𝜀 =1.1 ∙ ∙ 𝑊𝑒
𝐸 ∙ 𝑊𝑤 (4 − 3)
Where: h – overburden depth, ft
𝑊𝑤 – width of the web pillar, ft
Figure 4 - 2 Deformation of the Soft Roof Layer above a Highwall Mine Pillar
As the thickness of the soft roof layer is shrunk in the vertical direction by an amount of
𝑏 × ∆𝜀, it will be laterally squeezed into the mine entry by an amount of ∆𝑙 on each side of the
web pillar as shown in Figure 4-2. Under a reasonable assumption that the volume of the roof
layer before and after the deformation remains the same, the total lateral elongation can be
determined by using the following equation:
2∆𝑙 =1.1𝑊𝑒𝑊𝑤
𝐸𝑊𝑤 − 1.1𝑊𝑒 (4 − 4)
43
The elongation of the section of the roof layer above the web pillar (2∆𝑙) will induce an
additional sagging of the roof in the entry. The maximum roof deflection after considering the
lateral squeezing effect of the soft roof layer (𝑆′𝑚𝑎𝑥 ) will be:
𝑆′𝑚𝑎𝑥 = 𝑊𝑒
2
2
+ 𝑆𝑚𝑎𝑥2 +
∆𝑙
1.219
2
− 𝑊𝑒
2
2
(4 − 5)
Apparently, the squeezing effect on roof deflection depends on overburden depth,
Young’s modulus, entry width, pillar width and the thickness of the roof layer. Among them, the
Young’s modulus most heavily affects the maximum deflection. When the pillar squeezing effect
is considered, the determined 𝑆′𝑚𝑎𝑥 using Equation 4-5 can replace 𝑆𝑚𝑎𝑥 in Equation 4-2c for the
purpose of determining the roof deflection at a given point S(x).
In general, each layer in the roof strata is subjected to not only the layer weight itself, but
the load as a result of interaction effect between layers. Therefore, the interaction effect between
layers should be taken into consideration when assessing the stability of the roof entry. In order to
figure out the interaction effect on the stability of the rock layer in the immediate roof, the key
factor is to determine the load undertaken by the first immediate roof layer, which is defined as q
(Qian, et al., 2003). On the whole, it is assumed that there are still several thin and weak layers
above the first immediate roof layer. Additionally, the load exerted on layers is distributed
unevenly. However, in order to analyze the problems conveniently, it is assumed that the stress
exerted on each layer is distributed uniformly. Now take the first layer as an example to illustrate
the method of calculation of load q.
It is assumed that there are total of (m+1) layers in the roof strata. The thickness of each
layer is 𝑖 (i= 1,2, …,m), the unit weight of rock layer is 𝛾𝑖(i= 1,2, …,m), and the elastic modulus
is 𝐸𝑖 (i= 1,2, …,m). The number of layers that deform with the first layer is represented by n, as
shown in Figure 4-3. The first layer and the other n layers will deform simultaneously, forming a
composite beam. According to the theory of composite beams, the shear stress (Q) and bending
moment (M) of each cross section of a composite beam are the summation of the shear stress and
bending moment of each small cross section, shown in Equation4-6 and 4-7. According to the
theory of material mechanics, the curvature is 𝑘𝑖 = 1𝜌𝑖
(𝜌𝑖 is the radius of curvature), and the
relationship between the bending moment and curvature is shown in Equation 4-8.
44
𝑄 = 𝑄1 + 𝑄2 + ⋯ + 𝑄𝑛 (4 − 6)
𝑀 = 𝑀1 + 𝑀2 + ⋯𝑀𝑛 (4 − 7)
𝑘𝑖 =1
𝜌𝑖=
𝑀𝑖 𝑥𝐸𝑖𝐼𝑖
(4 − 8)
Figure 4 - 3Load Calculation of Roof Layers above a Highwall Entry
Due to the reason that these layers are combined together, the curvature between layers
should be same, leading to the redistribution of the bending moment of each layer. The
relationship between each layer is shown in Equation 4-9. The procedure to calculate the
magnitude of q is shown in the following equations from 4-10a to 4-10d. Finally, the load
undertaken by the first layer is derived, which is shown in Equation 4-10.
𝑀1
𝐸1𝐼1=
𝑀2
𝐸2𝐼2= ⋯ =
𝑀𝑛
𝐸𝑛 𝐼𝑛 (4 − 9)
𝑀𝑥 = 𝑀1 𝑥 + 𝑀2 𝑥 + ⋯ 𝑀𝑛 𝑥 (4 − 10𝑎)
𝑀𝑥 = 𝑀1 𝑥 1 +𝐸2𝐼2 + 𝐸3𝐼3 + ⋯𝐸𝑛 𝐼𝑛
𝐸1𝐼1 (4 − 10𝑏)
𝑀1 𝑥 =𝐸1𝐼1 ∙ 𝑀𝑥
𝐸1𝐼1 + 𝐸2𝐼2 + ⋯𝐸𝑛 𝐼𝑛 (4 − 10𝑐)
1
2
. . .
n.
. .
m
q
45
Due to 𝑑𝑀
𝑑𝑥= 𝑄,
𝑑𝑄
𝑑𝑥= 𝑞,
𝑞1 𝑥 =𝐸1𝐼1 ∙ 𝑞𝑥
𝐸1𝐼1 + 𝐸2𝐼2 + ⋯𝐸𝑛 𝐼𝑛 (4 − 10𝑑)
Where 𝑞𝑥 = 𝛾11 + 𝛾22 + ⋯+ 𝛾𝑛𝑛 ;
𝐼1 =𝑏1
3
12, 𝐼2 =
𝑏23
12, ⋯𝐼𝑛 =
𝑏𝑛3
12
After plugging 𝑞𝑥 , 𝐼1, 𝐼2 ⋯𝐼𝑛 into Equation 4-10d, the load undertaken by the first layer
is derived, as shown in Equation 4-10.
𝑞𝑛 1 =𝐸11
3(𝛾11 + 𝛾22 + ⋯ + 𝛾𝑛𝑛)
𝐸113 + 𝐸22
3 + ⋯ + 𝐸𝑛𝑛3 (4 − 10)
Table 4 - 1 Calculation Example Table
Layer Strata Density, lbs/ft3 Thickness, ft Young's Modulus, psi Tensile Strength, psi
1 Mudstone 120 0.2 7.50E+05 725
2 Mudstone 120 0.1 7.50E+05 725
3 Mudstone 120 0.1 7.50E+05 725
4 Mudstone 120 0.2 7.50E+05 725
The load undertaken by the first immediate roof layer depends on Young’s modulus, unit
weight of each rock layer, as well as thickness of each rock layer. Therefore, when the interaction
effect between rock layers is considered, the determined 𝑞𝑛 1 using Equation 4-10 can replace w
in Equation 4-1 for the purpose of determining the roof deflection. Table 4-1 provides parameters
to calculate the load undertaken by the first layer.
Load undertaken by the first layer:
𝑞1 = 𝛾1 × 1 = 120 × 0.2 144 = 0.167𝑝𝑠𝑖
Load undertaken by the first layer considering the interaction of second layer to the first
layer:
46
𝑞2 1 =𝐸11
3(𝛾11 + 𝛾22)
𝐸113 + 𝐸22
3 = 0.222𝑝𝑠𝑖
Load undertaken by the first layer considering the interaction of second and third layers
to the first layer:
𝑞3 1 =𝐸11
3(𝛾11 + 𝛾22 + 𝛾33)
𝐸113 + 𝐸22
3 + 𝐸333 = 0.267𝑝𝑠𝑖
Load undertaken by the first layer considering the interaction of second, third, and fourth
layers to the first layer:
𝑞4 1 =𝐸11
3(𝛾11 + 𝛾22 + 𝛾33 + 𝛾44)
𝐸113 + 𝐸22
3 + 𝐸333 + 𝐸44
3 = 0.222𝑝𝑠𝑖 < 𝑞3 1
The above calculation shows that the interaction of the second and third layers to the first
layer should be taken into consideration. Due to the relatively larger thickness of the fourth layer,
it is not likely to cause any impact to the first layer. Therefore, the load undertaken by the first
layer is 0.267 psi. If the fourth layer is sandstone, the load transfer process will be terminated as
well because of its larger Young’s modulus. Therefore, when encountering the relatively stronger
and thicker layer, the stress exerted on the first layer will be terminated by that layer, which
means that the failure process tends to be ended by that layer. It can also be concluded that if the
rock in the interburden is soft and the thickness of each layer is relatively thin, then it is quite
likely for the roof layers directly above the highwall mine entry to separate and sag sequentially
until they encounter relatively stronger and thicker layers.
Now, change the property of strata to sandstone. The density is 160 lbs/ft3, Young’s
modulus is 5.0E+06 psi, tensile strength is 600 psi, and the other conditions remain the same.
Following the same calculation procedure, the load undertaken by the first layer is 0.356 psi.
Under the same geometry condition, the load undertaken by the sandstone layer in the immediate
roof is larger than the load undertaken by the mudstone layer in the immediate roof. This is
because the density of sandstone is larger than that of mudstone. For the above example of
mudstone, if the layer thickness is changed to 0.05, 0.025, 0.025, and 0.05 ft respectively, the
load undertaken by the first layer is 0.067 psi. For the above example of sandstone, if the layer
thickness is also changed to 0.05, 0.025, 0.025, and 0.05 ft respectively, the load undertaken by
47
the first layer is 0.089 psi. From this, it should also be noted that the thinner the layer, the less
load would be undertaken by the first layer.
For coal measure rocks, the tensile strain should be more critically examined than the
compressive strain. In order to assess the tensile strain of the immediate entry roof, the critical
tensile strain values deducted from the subsidence monitoring programs performed on various
structures affected by mining operation subsidence are used. Based on the published literature and
our own subsidence studies, the critical tensile strain values for sandstone and mudstone are
2.0×10-3
ft/ft and 1.5×10-3
ft/ft, respectively.
For quick reference, Table 4-2 shows the physical and mechanical properties of a number
of common coal measure rocks in dry conditions. However, it should be pointed out that the
mechanical properties and strength of mudstone could be greatly affected by its moisture content.
Table 4 - 2 Physical and Mechanical Properties of Coal Measure Rocks in Dry Condition
(Zhao)
Rock
Type
Dry Density, lbs/ft3 UC Strength, psi Tensile Strength, psi Young's Modulus, psi
Poisson's
Ratio
Strain at
Failure, % Min Max Min Max Min Max Min Max
Sandstone 119 161 2,900 24,650 580 3,625 2.175E+06 7.250E+06 0.14 0.20
Shale 125 150 725 14,500 290 1,450 7.250E+05 4.350E+06 0.10
Mudstone 113 168 1,450 14,500 725 4,350 7.250E+05 1.015E+07 0.15 0.15
Limestone 167 170 4,350 36,250 870 3,625 2.900E+06 1.015E+07 0.30
A computer program has been developed in MS Excel based on the roof beam model.
The program can be used to assess the stability of an entry roof when it consists of soft and thin
rock layers. The input and the primary calculation page of the program is shown in Figure 4-4. In
the data input sections, the user should enter the geometric information of the highwall mine. The
ranges of the mechanical and physical properties of common coal measure rocks are given in
Table 4-2. The basic derived information is listed in the bottom portion of the table.
48
Figure 4 - 4 Input and Primary Calculation of the Roof Stability Assessment Program
Based on Equation 4-2c, the roof deflection profiles of 0.2-ft thick mudstone immediate
roof layer (E = 7.5×105 psi) are derived and shown in Figure 4-5. In the example, the overburden
depth, entry width, and web pillar width are 150, 11.5 and 4 ft, respectively. The rock density is
120 lbs/ft3. After taking interaction effect between layers into consideration, the load undertaken
by per unit length of a 1-inch thick layer is 0.267 psi. When only the gravity effect is considered,
the maximum roof deflection is 0.182 inches. When the pillar squeezing effect as well as
interaction effect between two layers are considered, the maximum roof deflection is about 0.374
inches. Therefore, the squeezing effect as well as interaction effect between layers should be
taken into consideration when evaluating the stability of the entry roof.
49
Figure 4 - 5 Roof Deflection Profiles for 0.2-ft Thick Mudstone Layer with and without
Lateral Squeezing Effect and Layers Interaction Effect
Figure 4 - 6 Roof Deflection Profiles for 0.2-ft Thick Mudstone and Sandstone Layers
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0 2 4 6 8 10 12
De
fle
ctio
n, i
nch
es
Distance from Pillar Edge, ft
Roof Layer Deflection
w/ S&I
w/o S&I
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0 2 4 6 8 10 12
De
fle
ctio
n, i
nch
es
Distance from Pillar Edge, ft
Roof Layer Deflection
Sandstone
Mudstone
50
Figure 4-6 shows the roof deflection profiles of 0.2-ft thick mudstone and sandstone
layers. The Young’s Modulus for sandstone layer is 5.0×106 psi, the density is 160 lbs/ft
3, and the
load undertaken by per unit length of a 1-inch thick layer is 0.356 psi based on the calculation
shown in the above example. Since the entry width, pillar width, and the thickness of each layer
for sandstone as well as mudstone layer are the same, the Young’s modulus, as well as the density
of rock layers, make big differences in this case. However, for a relatively thick layer spanning
over a relatively narrow entry, the estimate maximum deflections are 0.374 and 0.107 inches for
mudstone and sandstone respectively, which tends not to cause any stability problems for the
entry roof.
Figure 4-7 shows the roof deflection profiles for 0.05-ft thick mudstone and sandstone
layers. The load carried by per unit length of a 1-inch thick layer is 0.067 psi and 0.089 psi for
mudstone and sandstone, respectively, based on the above calculation analysis. From this figure,
the estimate maximum deflections are 4.703 and 0.939 inches for mudstone and sandstone,
respectively. It is easy to find that the roof deflection for mudstone is much higher than that of
sandstone, and such roof layer deflection could quite likely cause the stability problem for the
entry roof. Therefore, the existence of the relatively thin thickness and weak strata in the
immediate roof could cause potential stability problems for the entry roof.
Figure 4 - 7 Roof Deflection Profiles for 0.05-ft Thick Mudstone and Sandstone Layers
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
0 2 4 6 8 10 12
De
fle
ctio
n, i
nch
es
Distance from Pillar Edge, ft
Roof Layer Deflection
Sandstone
Mudstone
51
As it is difficult to precisely figure out the tensile strength of rock materials, tensile strain
is often used as a failure criterion, as shown in Table 4-2. Therefore, it is important to determine
the strain distribution on the top and bottom surfaces of the roof layer. Equation 4-11 can be used
to determine the surface strains on a sagging roof layer. In this equation, the first term is the
tensile strain caused by the beam elongation due to sagging, the second is the compressive strain
due to pillar squeezing, and the third term is due to beam bending. The positive sign (+) in front
of the third term represents the strain on the top surface of the layer and the negative sign (-)
represents the bottom surface. If the resulting strain is a positive one, it is in tension and
otherwise in compression. For coal measure rocks, the tensile strain should be more critically
examined than the compressive strain.
𝜀 𝑥 = [1.219 × ( 1 + 4 𝑆 ′
𝑚𝑎𝑥
𝑙
2
− 1) ] − (1.1𝑊𝑒𝑊𝑤
𝐸𝑊𝑤 − 1.1𝑊𝑒)
± [𝜌
6912𝐸𝑏 2𝑊𝑒
2 − 12𝑊𝑒𝑥 + 12𝑥2 ] (4 − 11)
As indicated by Equation 4-11, the thickness of the rock layer will greatly affect the
maximum strains at the pillar edge and at the center. The strain profiles of a 0.05-ft (0.6-inch)
thick mudstone and sandstone roof layers are shown in Figure 4-8 and Figure 4-9. The maximum
tensile strains on the top surface at the entry corner are 0.493% and 0.055% for mudstone and
sandstone respectively. The tensile strain for mudstone is larger than the failure strain shown in
Table 4-2 while the tensile strain for sandstone is smaller than the failure strain. Failure is most
likely to start at the entry corner, which is usually referred to as cutter roof phenomena. The
maximum tensile strains on the bottom surface at the center are 0.375% and 0.031% for mudstone
and sandstone, respectively. For mudstone, it also has the potential to cause roof failure.
Therefore, if the immediate entry roof strata consists of thinly bedded weak rock layers,
progressive and upward failure of the immediate mine roof is more likely to occur. This kind of
failure is not likely to terminate before encountering the relatively stronger and thicker layer.
Then the debris of failed roof layers, if in a large volume, can greatly hinder the mining
operations and potentially entrap the underground mining equipment. Therefore, for highwall
mining operations to be conducted in coal seams with thinly bedded roof strata, a correct decision
to cut some of the thin weak roof rock layers with the main coal seam can be greatly beneficial to
the mining operations.
52
Figure 4 - 8 Surface Strain Profiles of a 0.05-ft Mudstone Layer
Figure 4 - 9 Surface Strain Profiles of a 0.05-ft Sandstone Layer
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
0 2 4 6 8 10 12
Str
ain
, ft/
ft
Distance from Pillar Edge, ft
Strain on Layer Surfaces
Top Surface Bottom Surface
-6.0E-04
-4.0E-04
-2.0E-04
0.0E+00
2.0E-04
4.0E-04
6.0E-04
0 2 4 6 8 10 12
Str
ain
, ft/
ft
Distance from Pillar Edge, ft
Strain on Layer Surfaces
Top Surface Bottom Surface
53
Figure 4 - 10 Strain Profiles on a 0.2-ft Dry Mudstone Roof Layer
Figure 4-10 shows the predicted strain profile on the top and bottom surfaces of 0.2-ft
thick mudstone layer above an 11.5-ft wide entry. A positive strain means the roof layer is in
tension while a negative strain means the roof layer is in compression. Apparently, the maximum
tensile strains are located directly above the pillar edge. On the top surface, the maximum tensile
strain above the pillar edge is 0.0516%. On the bottom surface, the maximum tensile strain is
0.0139%, and occurs at the center of the entry. In comparison to the critical tensile strains in the
range from 0.15% to 0.20% (Table 4-2), the tensile strains are still too small to cause tensile
failure of these rock layers above the entry roof. It shows the maximum tensile strains directly
above the pillar edge are significantly smaller than those of 0.05-ft (2.4 inches) thick layers. The
Figure also shows that the magnitudes of the maximum tensile strain on the top surface at the
entry corner is much larger than that on the bottom surface at the entry center. Therefore, it is
quite likely that the layer begins to fail at the entry corner.
The stresses on the top and bottom surfaces can be determined by using the Equation 4-
11.
𝜍 𝑥 = 𝐸 ∙ 𝜀 𝑥 (4 − 12)
-1.2E-03
-1.0E-03
-8.0E-04
-6.0E-04
-4.0E-04
-2.0E-04
0.0E+00
2.0E-04
4.0E-04
6.0E-04
0 2 4 6 8 10 12
Str
ain
, ft/
ft
Distance from Pillar Edge, ft
Strain on Layer Surfaces
Top Surface Bottom Surface
54
Figure 4 - 11 Stress Profiles on the Top and Bottom Surfaces of 0.05-ft Mudstone Roof
Layer
Figure 4 - 12 Stress Profiles on the Top and Bottom Surfaces of 0.05-ft Sandstone Roof
Layer
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
0 2 4 6 8 10 12
Str
ess
, psi
Distance from Pillar Edge, ft
Stress on Layer Surfaces
Top Surface Bottom Surface
Critical Stress
-3,000
-2,000
-1,000
0
1,000
2,000
3,000
0 2 4 6 8 10 12
Str
ess
, psi
Distance from Pillar Edge, ft
Stress on Layer Surfaces
Top Surface Bottom Surface
Critical Stress
55
Figure 4-11 and Figure 4-12 show the stress profiles on the top and bottom surfaces of
0.05-ft thick mudstone and sandstone layers. The tensile strength of the mudstone (725 psi) and
sandstone (600psi) are also plotted in the figure. Figure 4-11 shows that both the maximum
tensile stresses on the top surface at the edges of the pillar (3,698 psi) and that on the bottom
surface at the entry center (2,809 psi) are larger than the critical mudstone tensile strength. Figure
4-12 shows that both the maximum tensile stresses on the top surface at the edges of the pillar
(2,738 psi) and that on the bottom surface at the entry center (1,554 psi) are larger than the critical
sandstone tensile strength. However, as the roof sagging is a gradual process, it is more likely for
the thinly bedded immediate roof rock layers to fail at the pillar edge in the form of cutter roof
than at the center of the entry in the form of tension cracks. This phenomenon of roof failure has
been frequently observed in underground coal mines. Therefore, the cutter roof in the highwall
mining is caused by tensile failure and not by shear failure as is commonly believed.
Figure 4 - 13 Stress Profiles on the Top and Bottom Surfaces of 0.2-ft Mudstone Roof Layer
The stress profile on the top and bottom surfaces of a 0.2-ft thick mudstone layer is
shown in Figure 4-13. For a 0.2-ft thick immediate roof layer, the maximum tensile stresses on
the top and bottom surfaces of the layer for mudstone are 387 and 104 psi. In dry conditions, this
-1,000
-800
-600
-400
-200
0
200
400
600
800
0 2 4 6 8 10 12
Str
ess
, psi
Distance from Pillar Edge, ft
Stress on Layer Surfaces
Top Surface Bottom Surface
Critical Stress
56
thicker mudstone layer is not likely to fail. However, when the mudstone becomes wet, its tensile
strength will be greatly reduced and it is possible to fail.
It should be noted that it is hard to keep a mine in dry conditions. When the mudstone is
wet, the mechanical properties of coal rocks, especially mudstone, claystone, and shale, could
change considerably. First, the uniaxial compressive strength (UCS) decreases as the moisture
content increases (Lashkaripour and Ajalloeian, 2000). Since the elastic modulus of sedimentary
rocks decreases with the uniaxial compressive strength (Chang, et al., 2006), the wetted rocks are
more deformable under the same loading condition. The rock tensile strength is generally linearly
proportional to its uniaxial compressive strength (Nazir, et al., 2013). Therefore, the tensile
strength of the roof rock layer at wet conditions should be much smaller than that at dry
conditions.
4.4 Pillar Stability for Highwall Mining in Single Coal Seam
As mentioned previously, the most important design task for a highwall mining operation
is to ensure that the pillars will not fail during and after the mining operation. Highwall pillars,
which are formed after driving parallel entries in the seam from the highwall, are long, narrow,
rectangular coal pillars left to support the overlying strata and the highwall. Since no artificial
supports are provided in the entries, stable pillars are essential for a successful highwall mining
operation. If the pillars collapse during active mining operations, it is quite likely for overburden
to cave into the mining entries and endanger the underground mining machines. This often
involves high risk of destabilizing the slope, burial of highwall cutting equipment, and a loss of
minable resources. Most seriously, large deformation or failure of the pillars at the mouth could
cause failure of the highwall. Pillar failure in a large contiguous area could also cause chimney or
trough types of subsidence events on the ground surface.
In order to avoid these adverse conditions for highwall mining operations, adequately
sized web and barrier pillars are the most fundamental to the overall stability and safety of the
highwall mining operations (Zipf, 2009). A systematic design of a highwall mining panel,
consisting of a number of web pillars bounded by two barrier pillars as shown in Figure 4-14, is
employed to achieve the required stability of the mine pillars and to maintain an acceptable
recovery ratio of coal reserves. The web pillar is relatively small and is only responsible for
carrying the tributary load. The barrier pillar should undertake the entire overburden load,
assuming that all of the web pillars in the two adjacent panels have failed completely in an
57
extreme case. In the design, the stability factors of 1.3 and 1.6 for the web and barrier pillars
under normal tributary load conditions are used, respectively. It should be noted that the
overburden depth at the mouth section of a highwall mine is normally much smaller than the
overburden depth along the remaining length. The pillar widths designed according to the average
or maximum overburden depths along the length of the entries should ensure pillar stability at the
mouth section. Under the extreme loading condition, when all web pillars in the adjacent panels
fail, the stability factor for the barrier pillars is set as 1.0.
Since the height of the web pillar is determined by the seam thickness, the width of the
web pillar needs to be designed rationally. From Equation 4-4in the last section, it can be found
that the width of the web pillars plays an important role in affecting the elongation of the roof
layer above the web pillar. Then, the maximum deflection, strain distribution, and stress
distribution of the immediate entry roof strata will be influenced by the width of the web pillar.
Therefore, after determining the width of the web pillar, a stability assessment of the entry roof
should be conducted based on the methods proposed in the previous section.
Figure 4 - 14 a Schematic for Highwall Mining Design (Luo, 2014)
58
4.5 Influence of Multi-Seam Mining Operations
It should be noted that between 20 and 40 percent of highwall mining operations are
carried out in closely spaced coal seams. The strata deformations and stress concentrations as a
result of past and current mining activities in the underlying or overlying coal seams could impact
the stability of the mine structures in the active highwall mine. When the thickness of the
interburden strata is smaller than one entry width, the multi-seam mining (MSM) interactions
could be potentially strong enough to impact the mining operations (Zipf, 2009). In such
scenarios, the MSM interactions have to be cautiously considered in the design and operations of
highwall mines.
In order to reduce the undesirable MSM interactions when the coal seams are closely
spaced, it is suggested to vertically align the highwall miner entries and pillars as shown in Figure
4-15. As such, if the soft and thin interburden is unable to carry its own weight and the weight of
the mining machine, downward mining sequence would be favored. In this scenario, water
accumulated in the previous mine entries in the upper seam(s) could potentially create some
issues for the mining operations in the underlying coal seam. Though, most of the industry
practices in ultra-close, multiple-seam highwalls mine the lower seam first, followed by the upper
seam (Peng, 2008). In this manner, the pit would be backfilled to build a platform to approach the
upper workable seam.
With the purpose of preventing web pillars from collapsing and highwall failures, it is
suggested that reducing the number of highwall mining entries between barrier pillars to five.
And if the mining heights in the seams are significantly different, the resulting sizes of the web
and barrier pillars for one seam could differ considerably from those in the other seams as well.
The larger pillar sizes should be adopted in the design for the pillar alignment vertically as it is
recommended in Figure 4-15. Additionally, if the interburden thickness is less than one highwall
miner entry width, stacked pillars are effectively a tall pillar with its height equal to the sum of
the height of the upper seam pillar, the lower seam pillar, and the interburden thickness (Peng,
2008).
When the interburden strata are adequately competent, the design of the web and barrier
pillars can be carried out provided that the highwall mining is conducted in each of the individual
coal seams alone. In the following part of this research, two models are constructed with
59
Examine2D and FLAC to provide examples to find the minimum thickness of interburden where
no interaction between the two coal seams is expected.
Figure 4 - 15 Suggested Layout for Highwall Mining in Closely Spaced Multiple Coal Seam
(Luo, 2014)
4.6 Model Development in Examine2D
In the model of Examine2D, the material being modeled is based on the following
assumptions:
(a)The geo-materials are homogenous, isotropic and linearly elastic;
(b)The whole model is established based on the assumption of hydrostatic situation;
(c)The whole model in Examine2D is constructed based on the assumption of plane
strain. This means that the modeled excavation is of infinite length normal to the plane section of
the analysis, which corresponds well with the highwall mining situation that the web and barrier
pillars are very long compared to their cross section dimensions.
60
The model is constructed based on the following mining and geological conditions:
Overburden depth for the lower coal seam: 170 ft
Mining height in both coal seams: 6 ft
Entry width: 11.5 ft (Entries in both seams are vertically aligned)
Number of the entries in a panel: 5
Multi-seam mining sequence: overmining
Interburden thickness: 11.5 ft
Web pillar thickness: 4 ft
Barrier pillar thickness: 10.6 ft
Density: 160 lbf/ft3
Poisson’s Ratio: 0.25
Young’s modulus: 2×106 psi,
Tensile strength: 122 psi
Cohesion: 183.3 psi
Internal friction angle: 28°
The failure trajectories and strength factor contours for 11.5 ft interburden thickness are
shown in Figure 4-16. In Examine2D, if the strength factor is less than one, this indicates the
material would fail under given stress conditions. In Figure 4-16, it is easy to note that there is
possibility of failure around the entries indicated by the orange regions according to the strength
factors in the upper corner of this figure. It is also easy to find that almost all the web pillars in
the panels are overstressed, and it is quite likely for them to fail under such loading conditions. In
order to determine the failure area of this model, failure trajectories are displayed at grid points
where the induced elastic stresses exceed the strength envelop of the material. From Figure 4-16,
two intersecting lines (an ―X‖) are found in the web pillars between entries. This means that the
overstress phenomenon has occurred and there are potential failures in the web pillars. Figure 4-
16 also shows a sign of the pressure arch effect that there are more failure trajectories for inner
panel web pillars than the pillars in the outer part of the panel which means that the possibility for
inner panel web pillars to fail is much higher than outer panel web pillars. This is because more
61
stress would be transferred to barrier pillars for the outer panel web pillars than the inner panel
web pillars. In particular, most of the web pillars in the inner part of panels are subjected to
extreme loading conditions, and they would fail as indicated by red color zones. However, due to
the relatively large size of barrier pillars, there should be no stability problems, which reduces the
potential for cascading pillar failure.
Figure 4 - 16 Failure Trajectories and Strength Factor Contours for 11.5ft Interburden
Thickness
The results derived from Figure 4-16 also verify the conclusion of Zip (2009) that when
the thickness of interburden is smaller than one entry width, the interaction caused by multi-seam
mining is capable of causing stability issues for highwall mining structures. In order to find out
the stable interburden thickness, the interburden thickness is changed in this model until no
failure trajectories are found around the web pillars. Finally, the stable interburden thickness of
20 ft is derived, as shown in Figure 4-17. The mining activities in the overlying coal seam will
not affect the stability of the underlying coal seam. Under such mining and geological conditions,
the pillar designs in each seam could be treated independently. However, it is still required to
keep the pillars in both seams as vertically aligned as possible. This is because when upper and
lower seam pillars are offset horizontally relative to one another, the interburden is also loaded by
stress concentrations under the upper pillar offset edge (Zhou and Haycocks, 1989). The high
62
concentration stresses developed on the offset rib-side have great potential to lead to cutter roof
failure in the interburden. In the Figure 4-18, the pillars in the upper seam are not columnized
with the pillars in the lower seam. It can be noted that the interaction between two seams under
offset pillar condition is much higher than that under columnized pillar condition. For these
geological and mining conditions, the interburden is stable and the mining activities in two seams
can be treated independently.
Figure 4 - 17 Failure Trajectories and Strength Factor Contours for 20ft Interburden
Thickness
63
Figure 4 - 18 Failure Trajectories and Strength Factor Contours for 20ft Interburden
Thickness under Offset Upper Pillars Condition
4.7 Model Development in FLAC
In this section, with the purpose of further evaluating the stability of interburden between
two seams, a FLAC model is constructed to explore whether the 20 ft thickness of interburden is
stable. In the meantime, the FLAC model is preformed to check out the stability of the entry roofs
and pillars and detect other potential failure mechanisms.
4.7.1 Model Assumption
The whole model is established based on the following assumptions:
(a) The model is set up based on a lithostatic stress field: vertical and horizontal stresses
are equal and based on overburden load (Vandergrift, et al., 2004);
(b) Both rock and coal are elastic and isotropic material; the calculated in-situ vertical
stress is equal to
𝜍𝑧 = 𝛾𝐻 (4 − 13)
64
Where 𝛾- the unit weight of the overlying rock
H - the depth below surface;
(c) To simulate the ground pressure of the miner, appropriate stress is applied in the last
entry in the upper seam based on the weight of the highwall miner.
4.7.2 Mining and Geology Conditions
In this model, two seams are extracted by highwall mining. Both coal seams are 6ft thick,
and the upper seam entries and lower seam entries are vertically lined up. The geometry of this
model is shown in Figure 4-19. The rock mechanics input parameters in the model are shown in
the following Table 4-3.
This model is constructed based on the following mining conditions:
Overburden depth: 170 ft
Entry width: 11.5 ft
Web pillar width: 4 ft
Number of entries: 5
Multi-seam mining sequence: overmining
Interburden thickness: 20 ft (Based on the results derived from Exaime2D)
65
Figure 4 - 19 Geometry of the Model
Table 4 - 3 Rock Mechanics Input Parameters in the Model
Inputs Coal Seam Weak Shale
Density (lb/ft3) 85 162
K ( Bulk Modulus) (lbf/ft2) 3.60E+07 1.92E+08
G ( Shear Modulus ) (lbf/ft2) 1.66E+07 1.15E+08
Cohesion (lbf/ft2) 3.89E+04 2.64E+04
Friction Angle (degree) 28 28
Tensile Strength (lbf/ft2) 5760 17568
Young’s Modulus (lbf/ft2) 4.32×10
7 2.88×108
Poisson’s Ratio 0.3 0.25
Ground Surface
144ft
100ft
20ft
Highwall Entry, 11.5ftWeb Pillar, 4ft
Coal Seam, 6ft
66
4.7.3 Boundary Conditions and Constitutive Model
Roller boundaries are applied along the sides and fixed boundaries are applied at the
bottom of the model. The left and right side of the model will be fixed in X direction, and the
bottom of the model is fixed both in X and Y directions. Gravitational forces are applied to the
zones, and allow the in-situ stresses to develop as they occur in nature.
The whole model is constructed with Mohr-Coulomb material, which exhibits an elastic-
plastic behavior. And the whole model created in FLAC is a plane-strain model in which the
block is considered to have an infinite length normal to the plane section of the analysis.
4.7.4 Model Development
The model is solved in three steps, which is similar to the highwall mining practice. In
the first step, the model is generated based on the simplified geological conditions and then run to
initial equilibrium state, which simulates the stress state of the virgin coal seam without mining
disturbance. In the second step, the entries in the lower seam are extracted. In the last step, the
entries in the upper seam are developed and appropriate stress is applied in the last entry in the
upper seam.
4.7.5 Calculation Analysis and Result Discussion
In order to analyze the initial equilibrium state, the 𝜍𝑦𝑦 stress contours and unbalanced
force figures are generated, as shown in Figure 4-20 and 4-21. From Figure 4-20, it is shown that
the 𝜍𝑦𝑦 stress increases with the increasing depth, which corresponds to the basic theory 𝜍𝑧 =
𝛾𝐻. The vertical stress is increasing linearly, and the value of the vertical stress at the surface is
zero, corresponding to the real situation. The largest unbalanced force is decreasing with the
increase of computation steps. When the steps are close to 5000, the largest unbalanced force is
close to zero, which means that it reaches the equilibrium state. After analyzing these two figures,
it can be concluded that the model is under initial equilibrium state, thus it is practical to perform
this model.
67
Figure 4 - 20 the Geostatic State of YY-Stress Contour
Figure 4 - 21 Maximum Unbalanced Force at Initial Equilibrium
68
The maximum and minimum principal stress distributions around highwall entries are
shown in Figures 4-22 and 4-23. In the lower seam, the largest value of the maximum principal
stress is 8.00E+04 lbf/ft2, and the largest value of the minimum principal stress is 4.00E+04
lbf/ft2. In the upper seam, the largest value of the maximum principal stress is 6.00E+04 lbf/ft
2,
and the largest value of the minimum principal stress is 3.00E+04 lbf/ft2. Therefore, both the
pillars in the upper and lower seams are stable. Through monitoring the stress around the last
entry in the upper seam, it is found that the effect of ground pressure of the miner is not apparent,
and the ground pressure of the miner causes no concern to the entry stability. It should also be
noted that the stress transfer effect between two seams is not serious. Thus, the interburden strata
will maintain stability during mining activities. Since the coal seams are stable after mining
activities, the mining of the upper seam should have little effect on the stability of the lower
seam. The minimum interburden thickness of 20 ft derived from Examine2D is verified. The
seam interaction will not pose potential stability problems to these multiple seam mining
activities. Through examining the stress above the entry roofs in the Figures 4-22 and 4-23, the
relatively low stresses are not likely to cause any stability problems. Therefore, there are no
stability problems for this highwall design under such geological and mining conditions.
Figure 4 - 22 Maximum Principal Stress around the Entries
69
Figure 4 - 23 Minimum Principal Stress around the Entries
Figure 4 - 24 Stress State around the Entries
70
Figure 4-24 shows the stress state around the highwall entries. In the beginning, the
excavation of entries will disturb the initial geostatic state of the virgin area. Because of the close
distance of the highwall entries, the induced stresses around each entry would interact with each
other. Then after all the entries are excavated, the induced stresses around the highwall entries
will have reached a new equilibrium state. From Figure 4-24, it can be found that all the pillars
and entry roofs are in elastic state except the web pillar in the middle of the panel in the upper
seam. This web pillar’s rib is in tension, which has potential for failure. There is a tendency for
rib spalling, but not to the degree that would suggest instability of the whole web pillar. From the
above discussions, this highwall mining design is rational and there should be no stability
problem for the whole highwall structure.
4.8 Highwall Mining Design Optimization
Like any other mining operations, enhancing the recovery ratio of coal reserves without
sacrificing mine safety is the primary goal for highwall mining design and operations. When
using the tributary design method, the recovery ratio of coal resources within a production panel
is restricted by many factors. Among these factors, the overburden depth is the main factor to
restrict the recovery ratio of coal reserves because the web pillars are required to carry the entire
tributary overburden load, namely half-way to each adjacent pillar and all the way to the surface,
with an acceptable safety factor. When the rock strata in the overburden are thick and competent,
the pressure arch theory is recommended for use in the systematic design of the highwall mine
panels, as shown in Figure 4-15. In this design system, the web pillars in the production panel are
left to only carry the overburden load under the pressure arch. Through this design system, a high
recovery ratio will be accomplished within the panel. The large barrier pillar, separating the
adjacent production panels, is designed to take on the extreme loading condition when the web
pillars in the adjacent production panel failed totally with the purpose of avoiding the cascading
pillars phenomena. Using a design optimization process, the overall recovery of coal reserves
from a highwall mining panel system can be increased without hindering the production practices.
In this suggested design method, the production panel and two adjacent barrier pillars are
combined as a system. In order to ensure the existence of a pressure arch within the competent
overburden strata, it is required to carefully select the width of the production panel. Under the
pressure arch loading condition, the web pillars in the panels just need to carry the overburden
load under the pressure arch. As a result, these pillars can be designed smaller than those
designed using traditional methods.
71
In this design system, the pressure arch concept is adopted in the panel pillars design.
When an opening is excavated, the weight of the ground above the opening will be transferred
outward to the strata around the opening, forming a distressed zone under the pressure arch (IME,
1936). The overburden strata under the pressure arch bend slightly and no longer undertake the
super-incumbent mass of strata. The pressure arch is considered to exist in every mining
excavation’s roof and the load of the superincumbent strata would be transferred to the pressure
arch two abutments (IME, 1949). Based on this pressure arch concept, the web pillars only need
to undertake the overburden load up to the pressure arch, and barrier pillars will absorb the load
from web pillars. The pressure arch formed over a production panel is depicted in Figure 4-25.
An ellipse function can be used to mathematically define the pressure arch, as shown in Equation
4-14.
𝑥2
𝑎2+
𝑦2
𝑏2= 1 (4 − 14)
In this method, with the angle of abutment pressure, α, the semi-minor and major axes of
the ellipses, a and b can be correlated, as shown in the Equation 4-15. Based on NIOSH research,
the abutment angle is selected as 21°, which is appropriate for US coal mines (Mark and Chase,
1997). Within the equation, a is half width of the pressure arch production panel. The height (b)
of the pressure arch can also be determined by the Equation 4-15.
𝑏 =𝑎
𝑡𝑎𝑛𝛼=
𝑊𝑝
2𝑡𝑎𝑛𝛼 (4 − 15)
In the meantime, the thickness of the competent strata (hc),within which the pressure arch
is still able to exist, can be used to determine the maximum allowable panel width (Wmax). From
Figure 4-25, the term hc can be determined by subtracting the thickness of unconsolidated
materials near the ground surface from the overburden depth. The height (b) of the pressure arch
located x distance away from the center of the production panel is then determined, as shown in
Equation 4-16.
𝑦 = 𝑎2 − 𝑥2
𝑡𝑎𝑛𝛼 (4 − 16)
72
Figure 4 - 25 Pressure Arch in Highwall Mine Design (Luo, 2014)
The transverse cross-section area under the pressure arch can be analytically determined
using Equation 4-14. The total overburden load carried by the web pillars per foot length can be
determined by Equation 4-17, considering the average density of the overburden strata to be 𝛾.
𝑃 =𝛾𝜋𝑊𝑝
2
8𝑡𝑎𝑛𝛼 (4 − 17)
The actual panel width should be chosen based on the Equation 4-18. When a proper
panel width, 𝑊𝑝 , is selected, N entries and (N-1) web pillars can be arranged within the panel.
𝑊𝑝 = 𝑁 ∙ 𝑊𝑒 + 𝑁 − 1 𝑊𝑤 (4 − 18)
The load-carrying capacity of each web pillar can be determined by using Mark-
Bieniawski pillar strength formula (Mark, 1995).
73
𝐶𝑝 = 144 ∙ 𝑊𝑤 ∙ 𝜍𝑖 0.64 + 0.54𝑊𝑤
𝑚 (4 − 19)
In the above equation, m is the mining height of the coal seam. In the Equation 4-19, 900
psi should be selected for the inside strength of the coal (𝜍𝑖) for US coal mines based on Mark
and Chase’s suggestion (1997). However, if clay-rich rock strata, capable of being weakened with
great extent when encountering water, are presented in the immediate floor and roof, a reduced
strength (𝜍𝑖 = 600 𝑝𝑠𝑖) of the coal can be chosen. The total load capacity of the web pillars in a
highwall mining production panel under the pressure arch can be determined by substituting the
Equation 4-19 into Equation 4-20.
𝐶 = 𝑁 − 1 𝐶𝑝 (4 − 20)
In order to ensure the stability of the production panel, it is very essential to select an
adequate average safety factor (SF) for web pillars during the design process, as shown in
Equation 4-21.
𝑆𝐹 =𝐶
𝑃 (4 − 21)
Since the entry width (𝑊𝑒) is either 9.5 or 11.5ft depending on the highwall miner being
used, the width (𝑊𝑤 ) of web pillars is the only unknown variable that needs to be determined
from Equation 4-21 after substituting in the corresponding equations (Equations 4-17, 4-18, and
4-19). Once 𝑊𝑤 is determined, the recovery ratio of the highwall mining production panel can be
determined as:
𝜂𝑝 =𝑁 ∙ 𝑊𝑒
𝑊𝑝 (4 − 22)
It should be pointed out that the design procedure for web pillars in a production panel
within the pressure arch should be an iterative process. First of all, a rational panel width ( 𝑊𝑝 )
should be selected, which is capable of fitting N entries and (N-1) web pillars within the panel as
shown in Figure 4-15. In the meantime, with the purpose of ensuring that the top of the pressure
arch still exists within the competent strata, the panel width should be maintained smaller than the
maximum allowable panel width ( 𝑊𝑚𝑎𝑥 ). With the purpose of increasing the recovery ratio
under the given conditions, the selection of the panel width should comply with a practical range
for highwall mining operations.
74
When the width of web pillars in the production panel are determined, the safety factor
for each web pillar should also be examined. The safety factor for each web pillar from the left
edge to the right edge of the production panel can be determined by using Equation 4-23. The
integral part in the following equation determines the size of the shaded area under the pressure
arch in Figure 4-25.
𝑆𝐹𝑖 =𝐶𝑝
𝛾
1
(
𝑊𝑝
2)2−𝑥2
𝑡𝑎𝑛𝛼𝑑𝑥
𝑖+1
2 𝑊𝑒+𝑖𝑊−
𝑊𝑝
2
(𝑖−1
2)𝑊𝑒+(𝑖−1)𝑊−
𝑊𝑝
2
𝑖 = 1,2, … , 𝑁 − 1 (4 − 23)
It is clear that the safety factor for each web pillars differs with the distance away from
the panel center. Figure 4-26 shows the resulting safety factors for each web pillar in a production
panel in which six entries and five web pillars are contained with an average web pillar safety
factor of 1.3 is selected. Apparently, the web pillars adjacent to the barrier pillars on both sides of
the production panel have larger safety factors than those near the center. If the resulting safety
factor is significantly lower than 1.0, a higher average safety factor for web pillars should be
selected and the whole highwall mining production panel should be redesigned.
Figure 4 - 26 Resulting Safety Factors for Web Pillars in a Production Panel
The barrier pillar should be designed with a much larger width than the web pillars in the
production panels. It should be designed with a safety factor of 1.0to undertake the entire
overburden load under the worst loading condition when all the web pillars in the adjacent
1.67
1.271.19
1.27
1.67
1 2 3 4 5
Web Pillar Safety Factors
75
highwall mining panels have failed, as presented by Zipf (2005). The load to be undertaken by the
barrier pillar is determined as the entire overburden depth within a distance of the summation of
the width of barrier pillar and the highwall mining panel (Equation 4-24). The load undertaking
capacity for the barrier pillars is determined using Equation 4-19 after replacing 𝑊𝑤 with the
barrier pillar width (𝑊𝑏).
𝑃𝑏 = ∙ 𝑊𝑝 + 𝑊𝑏 ∙ 𝛾 (4 − 24)
The overall recovery ratio of the production panel (𝜂)can be determined by Equation 4-
25.
𝜂 =𝑁 ∙ 𝑊𝑒
𝑊𝑝 + 𝑊𝑏 (4 − 25)
Equation 4-25 shows that the overall recovery ratio of a highwall mining panel is a
function of the production panel width, the barrier pillar width, and the web pillars width. In any
mining operations, one of the principal goals is to increase the recovery ratio of the resources
with the prerequisite of ensuring safe operation. Because of the relatively complicated equations
and some practical constraint (e.g., N has to be an integer number) presented in this section, it is
necessary to carry out the optimization design process with a designed program in order to
maximize the recovery ratio of coal resources. The optimization process begins with a panel
width that ensures the existence of the top of the pressure arch within the competent strata. In the
meantime, N highwall entries and (N-1) properly sized web pillars should be able to fit into the
panel. In order to satisfy the average safety factor for the web pillars, the determination of the
width of the production panel, number of entries, and width of web pillars should go through an
iterative process. Then, the barrier pillar is designed based on the determined panel width. In the
end, the overall recovery ratio can be determined. There could be many practical plans to design
the production panel if a large thickness of the competent overburden strata exist over the
production panel. In this case, the design with the largest recovery ratio of coal resource can be
discovered.
4.9 Benefit of Using the Highwall Mining Design Method
Similar to any other pillar design tasks, selecting a proper overburden depth is very
crucial to a highwall mine design. Since most highwall mining operations would start from the
benches left by contour mining operations, the overburden depth along the length of the
76
production entries could vary significantly. Zipf (2009) has suggested a high average depth (h) to
be computed from the design depth from the minimum (most likely at mouth section, 𝑚𝑖𝑛 ) and
maximum (𝑚𝑎𝑥 ) overburden depths in the following formula.
= 0.75 ∙ 𝑚𝑎𝑥 + 0.25 ∙ 𝑚𝑖𝑛 (4 − 26)
It should be noted that a very high weight in Equation 4-26 is assigned for the maximum
overburden depth and all sizes of pillars are designed based on this average design depth. It is
quite likely that the determined average depth is much larger than the overburden depth at the
highwall mouth section. Therefore, the selected safety factors for the highwall mine design would
be much smaller than the safety factors of pillars at the entry mouth section, which make the
pillars at the highwall mouth section much more stable than the remaining length of the highwall
entries. Higher stability of pillars at the mouth section is critical to prevent any highwall failures.
It is quite unlikely for web pillars to fail when designing the web and barrier pillars of a
highwall mining production panel according to the pressure arch concept and the above design
procedure with a safety factor of 1.3. Even if all the web pillars in a highwall mine panel have
collapsed, the stable barrier pillars adjacent to the panel can still allow the pressure arch to exist.
In this manner, any strata movements would be controlled under the pressure arch. Therefore, the
fundamental cause for surface subsidence, another principal safety issue for highwall mining
operations, has been solved.
4.10 Highwall Mine Design Programs
Three spreadsheet programs have been developed to design highwall mine pillars with
the proposed design methodologies and concepts. In this section, the detailed descriptions for the
highwall mine design programs are presented.
The first design program can be used to assess the roof stability if the thinly bedded weak
layer exists in the immediate roof of the coal seam. As proposed in the above section, it is quite
likely for the thinly bedded weak layer in the immediate entry roof to sag significantly under the
gravity effect, the pillar’s squeezing effect, as well as the interaction effect between rock layers. It
is possible for the tensile failures to occur either at the corner of the entry or at the middle of the
entry if the roof sagging is too large. The roof layers directly above the highwall mine entry
would separate and sag layer by layer until encountering the relatively stronger and thicker layers,
which tends to result in serious problems to highwall mining operations. Under such situations, it
77
is necessary to take some approaches to mine the thin and weak rock layers with the main coal
seam together. Therefore, in order to terminate such progressive roof failure, it is very important
to make correct decisions when selecting the particular rock layer as the immediate roof of the
highwall entries. In this design program, the maximum overburden depth along the highwall
entries should be selected with the purpose of ensuring the stability of the entry roof even under
the extreme loading condition.
In order to assess the pillar stability and panel design under single coal seam extraction
condition, the second program is developed based on the tributary area method. A screen capture
of the highwall mine pillar design program is shown in Figure 4-27. In this program, the width of
the web pillars is designed in following two sections: (1) the section at the entry mouth, and (2)
the remaining length of a production panel. At the entry mouth section, the height of the highwall
is selected as the overburden depth. Due to weathering, the in-situ strength of coal at the mouth
section should be lower than the in-situ strength of coal in deeper locations. By selecting a lower
coal strength at the mouth (800 psi) section for the highwall mine design, the pillar will be
designed more conservatively and then the stability of the highwall will be strengthened. For the
remaining length of the highwall mine entries, the design depth is an average depth calculated by
Equation 4-26. The normal in-situ strength (900 psi) should be selected in the design for the
remaining length of the highwall entries. The 800 psi coal strength can also be selected for
highwall mining operations to be conducted under shallow covers (e.g., less than 300 ft). The
pillar strength is computed using the Bieniawski formula (Equation 4-19) which takes the pillar
width-to-height ratio into consideration.
The results of this design program are shown in Figure 4-27. In the case of this figure, the
highwall height is 70 ft and the maximum depth for overburden is 146 ft. The mining height is 6.0
ft. The in-situ coal strengths in deeper locations and the mouth section are 800 and 700 psi,
respectively. The program suggests that the web and barrier pillars be 3.6 and 13.2 ft wide,
respectively. All the pillar safety factors meet the requirements and list in the bottom portion of
the figure. The safety factor for the web pillars at the entry mouth section is 1.6, which is capable
of ensuring the stability of the highwall operations. The overall coal recovery for a highwall mine
panel is about 69.0%.
In order to maximize the recovery ratio, a panel design optimization process program is
developed. Figure 4-28 shows a screen capture of the design program. In the design optimization
process, the critical parameter is the thickness of the competent strata above the mined coal seam.
78
This program requires the user to input trial values for the width of the production panel and the
maximum number of entries within one production panel. Through an optimization process, a
feasible design plane that is able to provide the best recovery ratio will be achieved. The Figure 4-
28 example adopts the same input parameters from those parameters in Figure 4-27. After
running the optimization program, it is shown that the width of the panel is 83.3ft, the number of
entries is six, and the width of the five web pillars is 2.9 ft. The width of the barrier pillar is 10.5
ft and the overall recovery ratio of the highwall mine panel is 73.6%, which is 4.6% higher than
that derived through the traditional design process shown in Figure 4-27.
As for multi-seam highwall mining operations, especially those operations conducted in
close proximity, it is suggested to vertically align the highwall entries and pillars, as shown in
Figure 4-15, with the purpose of reducing the adverse multi-seam mining interactions. If the
interburden strata are thick and competent, the pillar design in each seam can be treated
independently. This is due to the fact that the seam thickness and overburden depth are different
leading to different designed sizes of pillars. The design plan for a particular coal seam yielding
the largest web and barrier pillars should be selected for the multi-seam highwall mining
operations.
79
Figure 4 - 27 Screen Capture of the Highwall Mine Pillar Design Program
80
Figure 4 - 28 Design Optimization for Highwall Mining Panel Based on Pressure Arch
Concept
81
Chapter 5 Conclusions and Recommendations
5.1 Summary
Highwall mining is a relatively new mining method, which is the preferred, and often the
only feasible method to recover the coal reserves in the central Appalachian coalfields. These
coal seams used to be mined by mountain-top-removal, underground, contour, and auger mining
methods. Compared to the mountain-top removal mining method, highwall mining method could
significantly reduce environmental impacts. Compared to the contour and auger mining methods,
the new method can considerably increase productivity and the recovery ratio of coal resources.
Since it is a hybrid of surface and underground mining methods, it is safer and much more
productive than the underground mining method. Therefore, it is probable that more coal
operators will choose the highwall mining method to recover the high-value coals in Appalachian
coalfields.
By far, the greatest ground control safety concerns in highwall mining operations are rock
falls from the highwall and mining equipment entrapment underground. Generally, there are two
factors affecting the highwall mining operations, namely geologic structure and highwall
structure stability. As for geological constraints, particular precautions should be considered to
minimize the risk of failure associated with hillseams. In the highwall mine design, more efforts
should emphasize maintaining the stability of the mine roof and pillars to avoid safety hazards to
both personnel and mining equipment, ensuring a successful highwall mining operation.
The method to assess the stability of the entry roof is proposed by applying the beam
theory. When evaluating the stability of the entry roof, the gravity effect, the squeezing effect,
and the interaction effect between layers are taken into consideration. After comparing the
deflection, stress, and strain profiles of 0.05 ft sandstone and mudstone roof layers, it can be
concluded that the existence of relatively thin and weak layer in the immediate roof could cause
potential stability problems for the entry roof. Therefore, for highwall mining operations to be
conducted in coal seams with thinly bedded roof strata, a correct decision to cut some of the thin
weak roof rock layers with the main coal seam can be greatly beneficial to mining operations.
Then, the pressure arch concept is applied for the systematic design of the highwall mining
operation. Within this theory, the web pillars in the production panel are designed to only carry
the overburden load under the pressure arch. Then, barrier pillars are designed to undertake the
extreme loading condition when the web pillars in the adjacent production panel fail completely.
82
Through a design optimization process, the overall recovery of the coal reserves can be greatly
increased. For multi-seam highwall mining operations, the number of entries is recommended to
reduce to five and the largest web and barrier pillar sizes should be adopted in the design and
vertically aligned into seams.
Numerical modeling results obtained from Examine2D and FLAC models show that,
under given conditions in the thesis, the stable interburden thickness is 20 ft where no interaction
between the two coal seams is expected. The Examine2D and FLAC models also support the
claim that when the thickness of the interburden strata is smaller than one entry width, the multi-
seam mining (MSM) interactions could be potentially strong enough to influence the mining
operations. Furthermore, both the Examine2D and FLAC modeling efforts show that the pillars,
the roof, and the interburden would remain stable under these geology and mining conditions.
Additionally, the effect of ground pressure of the miner is not apparent, and it causes no concern
to stability.
In the end, three spreadsheet programs are developed for assessment of highwall mine
structures, for the design of the web and barrier pillars, and for the optimization design process,
based on the proposed design concepts and methodologies. In these programs, the pillars are
designed at the mouth section and the remaining length of the entry, respectively. Therefore, the
safety factors for the web and barrier pillars at the mouth section would be much higher than the
selected safety factors in the design, making the pillars at the mouth section much more stable
than the remaining length of the entries. Stable pillars will provide conditions for smooth mining
operations and avoid immediate and long-term mine subsidence on the ground surface.
5.2 Future Research Recommendations
Based on the investigations conducted in this research, the following work needs to be
updated:
(1) Future work should find the stable interburden under different overburden depth while
considering different properties of interburden.
(2) FLAC2D model should be modified to better simulate the practical geological and
mining conditions.
83
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About the Author
Education
Master of Science in Mining Engineering, August 2015, West Virginia University
Bachelor of Engineering in Mining Engineering, June, 2012, China university of Mining
and Technology
Experience
Graduate Research Assistant (Aug.2013 – Aug.2015)
Department of Mining Engineering, WVU
Graduate Research Assistant (Sep.2012 – Jun.2013)
Key Laboratory of Mining Pressure and Strata Control, CUMT
Intern at Zhengzhou Coal Industry, Ltd., Henan, China (Jun.2012 – Aug.2012)
Intern at Huaibei Mining Group, Anhui, China (Jun.2011 – Aug.2011)
Rewards
National Scholarship (2012)
Bucyrus Scholarship (2011)
Second Prize at National Mathematic Contest in Modeling for undergraduates (2011)
Professional Associations
Member of Society for Mining, Metallurgy, and Exploration (SME)
